diff --git a/mypyc/lib-rt/librt_internal.c b/mypyc/lib-rt/librt_internal.c
index 4f6e138c96f9..acbacba61ec9 100644
--- a/mypyc/lib-rt/librt_internal.c
+++ b/mypyc/lib-rt/librt_internal.c
@@ -584,14 +584,35 @@ read_int_internal(PyObject *data) {
     if (likely(first != LONG_INT_TRAILER)) {
         return _read_short_int(data, first);
     }
-    PyObject *str_ret = read_str_internal(data);
-    if (unlikely(str_ret == NULL))
+
+    // Long integer encoding -- byte length and sign, followed by a byte array.
+
+    // Read byte length and sign.
+    _CHECK_READ(data, 1, CPY_INT_TAG)
+    first = _READ(data, uint8_t)
+    Py_ssize_t size_and_sign = _read_short_int(data, first);
+    if (size_and_sign == CPY_INT_TAG)
         return CPY_INT_TAG;
-    PyObject* ret_long = PyLong_FromUnicodeObject(str_ret, 10);
-    Py_DECREF(str_ret);
-    if (ret_long == NULL)
+    bool sign = (size_and_sign >> 1) & 1;
+    Py_ssize_t size = size_and_sign >> 2;
+
+    // Construct an int object from the byte array.
+    _CHECK_READ(data, size, CPY_INT_TAG)
+    char *buf = ((BufferObject *)data)->buf;
+    PyObject *num = _PyLong_FromByteArray(
+        (unsigned char *)(buf + ((BufferObject *)data)->pos), size, 1, 0);
+    if (num == NULL)
         return CPY_INT_TAG;
-    return CPyTagged_StealFromObject(ret_long);
+    ((BufferObject *)data)->pos += size;
+    if (sign) {
+        PyObject *old = num;
+        num = PyNumber_Negative(old);
+        Py_DECREF(old);
+        if (num == NULL) {
+            return CPY_INT_TAG;
+        }
+    }
+    return CPyTagged_StealFromObject(num);
 }
 
 static PyObject*
@@ -609,22 +630,81 @@ read_int(PyObject *self, PyObject *const *args, size_t nargs, PyObject *kwnames)
     return CPyTagged_StealAsObject(retval);
 }
 
+
+static inline int hex_to_int(char c) {
+    if (c >= '0' && c <= '9')
+        return c - '0';
+    else if (c >= 'a' && c <= 'f')
+        return c - 'a' + 10;
+    else
+        return c - 'A' + 10;  // Assume valid hex digit
+}
+
 static inline char
 _write_long_int(PyObject *data, CPyTagged value) {
-    // TODO(jukka): write a more compact/optimal format for arbitrary length ints.
     _CHECK_SIZE(data, 1)
     _WRITE(data, uint8_t, LONG_INT_TRAILER)
     ((BufferObject *)data)->end += 1;
+
+    PyObject *hex_str = NULL;
     PyObject* int_value = CPyTagged_AsObject(value);
     if (unlikely(int_value == NULL))
-        return CPY_NONE_ERROR;
-    PyObject *str_value = PyObject_Str(int_value);
+        goto error;
+
+    hex_str = PyNumber_ToBase(int_value, 16);
+    if (hex_str == NULL)
+        goto error;
     Py_DECREF(int_value);
-    if (unlikely(str_value == NULL))
-        return CPY_NONE_ERROR;
-    char res = write_str_internal(data, str_value);
-    Py_DECREF(str_value);
-    return res;
+    int_value = NULL;
+
+    const char *str = PyUnicode_AsUTF8(hex_str);
+    if (str == NULL)
+        goto error;
+    Py_ssize_t len = strlen(str);
+    bool neg;
+    if (str[0] == '-') {
+        str++;
+        len--;
+        neg = true;
+    } else {
+        neg = false;
+    }
+    // Skip the 0x hex prefix.
+    str += 2;
+    len -= 2;
+
+    // Write bytes encoded length and sign.
+    Py_ssize_t size = (len + 1) / 2;
+    Py_ssize_t encoded_size = (size << 1) | neg;
+    if (encoded_size <= MAX_FOUR_BYTES_INT) {
+        if (_write_short_int(data, encoded_size) == CPY_NONE_ERROR)
+            goto error;
+    } else {
+        PyErr_SetString(PyExc_ValueError, "int too long to serialize");
+        goto error;
+    }
+
+    // Write absolute integer value as byte array in a variable-length little endian format.
+    int i;
+    for (i = len; i > 1; i -= 2) {
+        if (write_tag_internal(
+                data, hex_to_int(str[i - 1]) | (hex_to_int(str[i - 2]) << 4)) == CPY_NONE_ERROR)
+            goto error;
+    }
+    // The final byte may correspond to only one hex digit.
+    if (i == 1) {
+        if (write_tag_internal(data, hex_to_int(str[i - 1])) == CPY_NONE_ERROR)
+            goto error;
+    }
+
+    Py_DECREF(hex_str);
+    return CPY_NONE;
+
+  error:
+
+    Py_XDECREF(int_value);
+    Py_XDECREF(hex_str);
+    return CPY_NONE_ERROR;
 }
 
 static char
diff --git a/mypyc/test-data/run-classes.test b/mypyc/test-data/run-classes.test
index 04bbed78b318..09655cf35d6b 100644
--- a/mypyc/test-data/run-classes.test
+++ b/mypyc/test-data/run-classes.test
@@ -2804,13 +2804,36 @@ def test_buffer_int_size() -> None:
 
 def test_buffer_int_powers() -> None:
     # 0, 1, 2 are tested above
-    for p in range(2, 9):
+    for p in range(2, 200):
         b = Buffer()
         write_int(b, 1 << p)
+        write_int(b, (1 << p) - 1)
         write_int(b, -1 << p)
+        write_int(b, (-1 << p) + 1)
         b = Buffer(b.getvalue())
         assert read_int(b) == 1 << p
+        assert read_int(b) == (1 << p) - 1
         assert read_int(b) == -1 << p
+        assert read_int(b) == (-1 << p) + 1
+
+def test_positive_long_int_serialized_bytes() -> None:
+    b = Buffer()
+    n = 0x123456789ab
+    write_int(b, n)
+    x = b.getvalue()
+    # Two prefix bytes, followed by little endian encoded integer in variable-length format
+    assert x == b"\x0f\x2c\xab\x89\x67\x45\x23\x01"
+    b = Buffer(x)
+    assert read_int(b) == n
+
+def test_negative_long_int_serialized_bytes() -> None:
+    b = Buffer()
+    n = -0x123456789abcde
+    write_int(b, n)
+    x = b.getvalue()
+    assert x == b"\x0f\x32\xde\xbc\x9a\x78\x56\x34\x12"
+    b = Buffer(x)
+    assert read_int(b) == n
 
 def test_buffer_str_size() -> None:
     for s in ("", "a", "a" * 117):
@@ -2835,6 +2858,8 @@ test_buffer_roundtrip()
 test_buffer_int_size()
 test_buffer_str_size()
 test_buffer_int_powers()
+test_positive_long_int_serialized_bytes()
+test_negative_long_int_serialized_bytes()
 
 def test_buffer_basic_interpreted() -> None:
     b = Buffer(b"foo")