diff --git a/src/libnix/exec.cc b/src/libnix/exec.cc
index a51b605d8..4934712f9 100644
--- a/src/libnix/exec.cc
+++ b/src/libnix/exec.cc
@@ -16,14 +16,14 @@ static string pathNullDevice = "/dev/null";
 
 /* Run a program. */
 void runProgram(const string & program, 
-    const Strings & args, const Environment & env)
+    const Strings & args, const Environment & env,
+    const string & logFileName)
 {
     /* Create a log file. */
-    string logFileName = nixLogDir + "/run.log";
     string logCommand = 
 	verbosity >= lvlDebug 
-	? "tee -a "  + logFileName + " >&2"
-	: "cat >> " + logFileName;
+	? "tee "  + logFileName + " >&2"
+	: "cat > " + logFileName;
     /* !!! auto-pclose on exit */
     FILE * logFile = popen(logCommand.c_str(), "w"); /* !!! escaping */
     if (!logFile)
diff --git a/src/libnix/exec.hh b/src/libnix/exec.hh
index 8d410e404..fc5bd6ac8 100644
--- a/src/libnix/exec.hh
+++ b/src/libnix/exec.hh
@@ -15,7 +15,8 @@ typedef map<string, string> Environment;
 
 /* Run a program. */
 void runProgram(const string & program, 
-    const Strings & args, const Environment & env);
+    const Strings & args, const Environment & env,
+    const string & logFileName);
 
 
 #endif /* !__EXEC_H */
diff --git a/src/libnix/normalise.cc b/src/libnix/normalise.cc
index 196fcad2e..0ce38d68a 100644
--- a/src/libnix/normalise.cc
+++ b/src/libnix/normalise.cc
@@ -164,7 +164,8 @@ Path normaliseNixExpr(const Path & _nePath, PathSet pending)
 
         /* Run the builder. */
         msg(lvlChatty, format("building..."));
-        runProgram(ne.derivation.builder, ne.derivation.args, env);
+        runProgram(ne.derivation.builder, ne.derivation.args, env,
+            nixLogDir + "/" + baseNameOf(nePath));
         msg(lvlChatty, format("build completed"));
         
     } else