diff --git a/endian_swap.h b/endian_swap.h
index a462f9a..aec6295 100644
--- a/endian_swap.h
+++ b/endian_swap.h
@@ -3,6 +3,16 @@
 
 #include <stdint.h>
 #include <inttypes.h>
+#include "assert_helpers.h"
+
+#ifdef BOWTIE_64BIT_INDEX
+#   define endianSwapU(x) endianSwapU64(x)
+#   define endianSwapI(x) endianSwapI64(x)
+#else
+#   define endianSwapU(x) endianSwapU32(x)
+#   define endianSwapI(x) endianSwapI32(x)
+#endif
+
 
 /**
  * Return true iff the machine running this program is big-endian.
@@ -54,25 +64,55 @@ static inline int32_t endianSwapI32(int32_t i) {
 }
 
 /**
- * Convert uint32_t argument to the specified endianness.  It's assumed
+ * Return copy of int64_t argument with byte order reversed.
+ */
+static inline int64_t endianSwapI64(int64_t u) {
+        int64_t tmp = 0;
+        tmp |= ((u >> 56) & (0xffull <<  0));
+        tmp |= ((u >> 40) & (0xffull <<  8));
+        tmp |= ((u >> 24) & (0xffull << 16));
+        tmp |= ((u >>  8) & (0xffull << 24));
+        tmp |= ((u <<  8) & (0xffull << 32));
+        tmp |= ((u << 24) & (0xffull << 40));
+        tmp |= ((u << 40) & (0xffull << 48));
+        tmp |= ((u << 56) & (0xffull << 56));
+        return tmp;
+}
+
+/**
+ * Convert uint32_t/uint64_t argument to the specified endianness.  It's assumed
  * that u currently has the endianness of the current machine.
  */
-static inline uint32_t endianizeU32(uint32_t u, bool toBig) {
+template <typename T>
+static inline T endianizeU(T u, bool toBig) {
 	if(toBig == currentlyBigEndian()) {
 		return u;
 	}
-	return endianSwapU32(u);
+	if(sizeof(T) == 4) {
+		return endianSwapU32((uint32_t)u);
+	} else if(sizeof(T) == 8) {
+		return endianSwapU64((uint64_t)u);
+	} else {
+		assert(false);
+	}
 }
 
 /**
- * Convert int32_t argument to the specified endianness.  It's assumed
+ * Convert int32_t/int64_t argument to the specified endianness.  It's assumed
  * that u currently has the endianness of the current machine.
  */
-static inline int32_t endianizeI32(int32_t i, bool toBig) {
+template <typename T>
+static inline T endianizeI(T i, bool toBig) {
 	if(toBig == currentlyBigEndian()) {
 		return i;
 	}
-	return endianSwapI32(i);
+	if(sizeof(T) == 4) {
+		return endianSwapI32((int32_t)i);
+	} else if(sizeof(T) == 8) {
+		return endianSwapI64((int64_t)i);
+	} else {
+		assert(false);
+	}
 }
 
 #endif
diff --git a/word_io.h b/word_io.h
index 1ec051e..b2752a3 100644
--- a/word_io.h
+++ b/word_io.h
@@ -9,131 +9,115 @@
 #include "btypes.h"
 
 /**
- * Write a 32-bit unsigned to an output stream being careful to
+ * Write a 32/64 bit unsigned to an output stream being careful to
  * re-endianize if caller-requested endianness differs from current
  * host.
  */
-static inline void writeU32(std::ostream& out, uint32_t x, bool toBigEndian) {
-	uint32_t y = endianizeU32(x, toBigEndian);
-	out.write((const char*)&y, 4);
+template <typename T>
+static inline void writeU(std::ostream& out, T x, bool toBigEndian) {
+	T y = endianizeU<T>(x, toBigEndian);
+	out.write((const char*)&y, sizeof(T));
 }
 
 /**
- * Write a 32-bit unsigned to an output stream using the native
+ * Write a 32/64 bit unsigned to an output stream using the native
  * endianness.
  */
-static inline void writeU32(std::ostream& out, uint32_t x) {
-	out.write((const char*)&x, 4);
+template <typename T>
+static inline void writeU(std::ostream& out, T x) {
+	out.write((const char*)&x, sizeof(T));
 }
 
 /**
- * Write a 32-bit signed int to an output stream being careful to
+ * Write a 32/64 bit signed int to an output stream being careful to
  * re-endianize if caller-requested endianness differs from current
  * host.
  */
-static inline void writeI32(std::ostream& out, int32_t x, bool toBigEndian) {
-	int32_t y = endianizeI32(x, toBigEndian);
-	out.write((const char*)&y, 4);
+template <typename T>
+static inline void writeI(std::ostream& out, T x, bool toBigEndian) {
+	T y = endianizeI<T>(x, toBigEndian);
+	out.write((const char*)&y, sizeof(T));
 }
 
 /**
- * Write a 32-bit unsigned to an output stream using the native
+ * Write a 32/64 bit unsigned to an output stream using the native
  * endianness.
  */
-static inline void writeI32(std::ostream& out, int32_t x) {
-	out.write((const char*)&x, 4);
+template <typename T>
+static inline void writeI(std::ostream& out, T x) {
+	out.write((const char*)&x, sizeof(T));
 }
 
-/**
- * Read a 32-bit unsigned from an input stream, inverting endianness
- * if necessary.
- */
-static inline uint32_t readU32(std::istream& in, bool swap) {
-	uint32_t x;
-	in.read((char *)&x, 4);
-	assert_eq(4, in.gcount());
+template <typename T>
+static inline T readU(std::istream& in, bool swap) {
+	T x;
+	in.read((char *)&x, sizeof(T));
+	assert_eq(sizeof(T), in.gcount());
 	if(swap) {
-		return endianSwapU32(x);
+		if(sizeof(T) == 4) {
+			return endianSwapU32(x);
+		} else if(sizeof(T) == 8) {
+			return endianSwapU64(x);
+		} else {
+			assert(false);
+		}
 	} else {
 		return x;
 	}
 }
 
-/**
- * Read a 32-bit unsigned from a file descriptor, optionally inverting
- * endianness.
- */
-static inline uint32_t readU32(int in, bool swap) {
-	uint32_t x;
-	if(read(in, (void *)&x, 4) != 4) {
-		assert(false);
-	}
-	if(swap) {
-		return endianSwapU32(x);
-	} else {
-		return x;
-	}
-}
 
-/**
- * Read a 32-bit unsigned from a FILE*, optionally inverting
- * endianness.
- */
-static inline uint32_t readU32(FILE* in, bool swap) {
-	uint32_t x;
-	if(fread((void *)&x, 1, 4, in) != 4) {
+template <typename T>
+static inline T readU(FILE* in, bool swap) {
+	T x;
+	if(fread((void *)&x, 1, sizeof(T), in) != sizeof(T)) {
 		assert(false);
 	}
 	if(swap) {
-		return endianSwapU32(x);
-	} else {
-		return x;
-	}
-}
-
-
-/**
- * Read a 32-bit signed from an input stream, inverting endianness
- * if necessary.
- */
-static inline int32_t readI32(std::istream& in, bool swap) {
-	int32_t x;
-	in.read((char *)&x, 4);
-	assert_eq(4, in.gcount());
-	if(swap) {
-		return endianSwapI32(x);
+		if(sizeof(T) == 4) {
+			return endianSwapU32(x);
+		} else if(sizeof(T) == 8) {
+			return endianSwapU64(x);
+		} else {
+			assert(false);
+		}
 	} else {
 		return x;
 	}
 }
 
-/**
- * Read a 32-bit unsigned from a file descriptor, optionally inverting
- * endianness.
- */
-static inline uint32_t readI32(int in, bool swap) {
-	int32_t x;
-	if(read(in, (void *)&x, 4) != 4) {
-		assert(false);
-	}
+template <typename T>
+static inline T readI(std::istream& in, bool swap) {
+	T x;
+	in.read((char *)&x, sizeof(T));
+	assert_eq(sizeof(T), in.gcount());
 	if(swap) {
-		return endianSwapI32(x);
+		if(sizeof(T) == 4) {
+			return endianSwapI32(x);
+		} else if(sizeof(T) == 8) {
+			return endianSwapI64(x);
+		} else {
+			assert(false);
+		}
 	} else {
 		return x;
 	}
 }
 
-/**
- * Read a 32-bit unsigned from a FILE*, optionally inverting
- * endianness.
- */
-static inline uint32_t readI32(FILE* in, bool swap) {
-	int32_t x;
-	if(fread((void *)&x, 1, 4, in) != 4) {
+template <typename T>
+static inline T readI(FILE* in, bool swap) {
+	T x;
+	if(fread((void *)&x, 1, sizeof(T), in) != sizeof(T)) {
 		assert(false);
 	}
 	if(swap) {
-		return endianSwapI32(x);
+		if(sizeof(T) == 4) {
+			return endianSwapI32(x);
+		} else if(sizeof(T) == 8) {
+			return endianSwapI64(x);
+		} else {
+			assert(false);
+		}
 	} else {
 		return x;
 	}