diff --git a/src/main/scala/scoverage/IOUtils.scala b/src/main/scala/scoverage/IOUtils.scala
index 78d5b663..0b41ad6d 100644
--- a/src/main/scala/scoverage/IOUtils.scala
+++ b/src/main/scala/scoverage/IOUtils.scala
@@ -14,10 +14,13 @@ object IOUtils {
   def clean(dataDir: String): Unit = clean(new File(dataDir))
 
   def coverageFile(dataDir: File): File = coverageFile(dataDir.getAbsolutePath)
-  def coverageFile(dataDir: String): File = new File(dataDir + "/" + CoverageFileName)
+  def coverageFile(dataDir: String): File = new File(dataDir, CoverageFileName)
 
+  /**
+   * @return the measurement file for the current thread.
+   */
   def measurementFile(dataDir: File): File = measurementFile(dataDir.getAbsolutePath)
-  def measurementFile(dataDir: String): File = new File(dataDir + "/" + MeasurementsPrefix + Thread.currentThread.getId)
+  def measurementFile(dataDir: String): File = new File(dataDir, MeasurementsPrefix + Thread.currentThread.getId)
 
   def findMeasurementFiles(dataDir: String): Array[File] = findMeasurementFiles(new File(dataDir))
   def findMeasurementFiles(dataDir: File): Array[File] = dataDir.listFiles(new FileFilter {
@@ -141,4 +144,4 @@ object IOUtils {
     reader.close()
     coverage
   }
-}
\ No newline at end of file
+}
diff --git a/src/main/scala/scoverage/Invoker.scala b/src/main/scala/scoverage/Invoker.scala
index 0ef0645a..cf870691 100644
--- a/src/main/scala/scoverage/Invoker.scala
+++ b/src/main/scala/scoverage/Invoker.scala
@@ -5,6 +5,8 @@ import java.io.FileWriter
 /** @author Stephen Samuel */
 object Invoker {
 
+  val threadFile = new ThreadLocal[FileWriter]
+
   /**
    * We record that the given id has been invoked by appending its id to the coverage
    * data file.
@@ -23,9 +25,12 @@ object Invoker {
   def invoked(id: Int, dataDir: String) = {
     // Each thread writes to a separate measurement file, to reduce contention
     // and because file appends via FileWriter are not atomic on Windows.
-    val file = IOUtils.measurementFile(dataDir)
-    val writer = new FileWriter(file, true)
-    writer.append(id.toString + ';')
-    writer.close()
+    var writer = threadFile.get()
+    if (writer == null) {
+      val file = IOUtils.measurementFile(dataDir)
+      writer = new FileWriter(file, true)
+      threadFile.set(writer)
+    }
+    writer.append(id.toString + ';').flush()
   }
-}
\ No newline at end of file
+}