diff --git a/pathdiscov/drop_mapped.py b/pathdiscov/drop_mapped.py
new file mode 100644
index 00000000..9a03745f
--- /dev/null
+++ b/pathdiscov/drop_mapped.py
@@ -0,0 +1,44 @@
+
+
+def _discard(_saved, _dropped):
+    with open(_saved, 'r'), open(_dropped, 'r') as saved, dropped:
+        try:
+            dropped_id = next(dropped)
+        except StopIteration:
+            warnings.warn("discard file %s was empty.")
+            return None
+            # just symlink
+        for read in SeqIO.parse(saved, 'fastq'):
+            if str(read.id) == dropped_id:
+                # read gets filtered because its ID was in discard so don't yield
+                dropped_id = next(dropped)
+            else:
+                yield read
+def discard(saved, disc, outpath):
+    res = discard(saved, disc)
+    if res:
+        SeqIO.write(res, outpath, 'fastq')
+    else:
+         os.symlink(os.path.abspath(saved), os.path.abspath(outpath))
+
+def main():
+    parser = argparse.ArgumentParser(
+        description = 'Drop discarded reads from the other pair.'
+    )
+    parser.add_argument(
+        '--saved',
+        help='The file with the unmapped reads.'
+    )
+    parser.add_argument(
+        '--other-discard',
+        help='The discard file for the *opposite* member of the pair.
+        If --saved is R1, this should be R2.discard, etc.'
+    )
+    parser.add_argument(
+        '--out',
+        help='The output path'
+    )
+    args = parser.parse_args()
+    discard(args.saved, args.other_discard, args.out)
+
+
diff --git a/pathdiscov/host_map/host_map.pl b/pathdiscov/host_map/host_map.pl
index 0f0d62d7..1ecd3866 100755
--- a/pathdiscov/host_map/host_map.pl
+++ b/pathdiscov/host_map/host_map.pl
@@ -306,8 +306,7 @@
 		if ( defined($hoh{$command}{$mate}) && -s $hoh{$command}{$mate} )
 		{
 			# update mate to new output
-			$hoh{$command}{$mate} = "$outputdir/map_$j/$mate.unmap.fastq";										
-			system("ln -sf map_$j/$mate.unmap.fastq $outputdir/host_map_$run_iteration.$mate");
+			$hoh{$command}{$mate} = "$outputdir/map_$j/$mate.unmap.fastq.tmp";										
 		
 			my $cmd = "linecount $hoh{$command}{$mate} $mapper_name_list[$i] $mate.count 1 1";
 			print "[cmd] ",$cmd,"\n";
@@ -315,10 +314,23 @@
 			
 			# (take this out of loop for efficiency - only needs to be done on the final iteration)
 			# get discard IDs - i.e., the reads filtered in this step
-			my $cmd = "fastaq_tools_diff.exe --fastq $hoh{$command}{$mate.\"input\"} --fastq $hoh{$command}{$mate} > $mate.discard";
+			my $cmd = "fastaq_tools_diff.exe --fastq $hoh{$command}{$mate.\"input\"} --fastq $hoh{$command}{$mate} | sort > $mate.discard";
 			verbose_system($cmd);
 		}
 	}
+
+    my $cmd = "drop_mapped --saved $hoh{$command}{"R1"} --dropped "R2".discard --out $mapout" 
+
+    verbose_system($cmd);
+
+    my $cmd = "drop_mapped --saved $hoh{$command}{"R2"} --dropped "R1".discard --out $mapout" 
+
+    verbose_system($cmd);
+
+  foreach my $mate (@mates) 
+  {
+	system("ln -sf map_$j/$mate.unmap.fastq $outputdir/host_map_$run_iteration.$mate");
+        }
 }
 			
 # --------------------------------------
diff --git a/setup.py b/setup.py
index 2d3e0fb2..85cbafbb 100644
--- a/setup.py
+++ b/setup.py
@@ -282,6 +282,7 @@ def run_tests(self):
             'sff2fastq = pathdiscov.sff2fastq:main',
             'step1 = pathdiscov.stages.step1:main',
             'get_blast_reads = pathdiscov.get_blast_reads:main',
+            'drop_mapped = pathdiscov.drop_mapped:main'
         ],
     },
     # These all get copied to our installation's bin folder for us