diff --git a/engine/src/main/java/org/archive/crawler/frontier/precedence/HighestUriQueuePrecedencePolicy.java b/engine/src/main/java/org/archive/crawler/frontier/precedence/HighestUriQueuePrecedencePolicy.java
index e112c742..82fd6e63 100644
--- a/engine/src/main/java/org/archive/crawler/frontier/precedence/HighestUriQueuePrecedencePolicy.java
+++ b/engine/src/main/java/org/archive/crawler/frontier/precedence/HighestUriQueuePrecedencePolicy.java
@@ -18,11 +18,11 @@
  */
 package org.archive.crawler.frontier.precedence;
 
-import java.io.PrintWriter;
-
-import org.archive.crawler.frontier.WorkQueue;
-import org.archive.modules.CrawlURI;
-import org.archive.modules.fetcher.FetchStats.Stage;
+import java.io.PrintWriter;
+
+import org.archive.crawler.frontier.WorkQueue;
+import org.archive.modules.CrawlURI;
+import org.archive.modules.fetcher.FetchStats.Stage;
 import org.archive.util.Histotable;
 
 /**
@@ -65,14 +65,18 @@ public class HighestUriQueuePrecedencePolicy extends BaseQueuePrecedencePolicy {
         public void tally(CrawlURI curi, Stage stage) {
             switch(stage) {
             case SCHEDULED:
-                // enqueued
-                enqueuedCounts.tally(curi.getPrecedence());
+                // enqueued
+                synchronized (enqueuedCounts) {
+                    enqueuedCounts.tally(curi.getPrecedence());
+                }
                 break;
             case SUCCEEDED:
             case DISREGARDED:
             case FAILED:
                 // dequeued
-                enqueuedCounts.tally(curi.getPrecedence(),-1);
+                synchronized (enqueuedCounts) {
+                    enqueuedCounts.tally(curi.getPrecedence(), -1);
+                }
                 break;
             case RETRIED:
                 // do nothing, already tallied
@@ -85,9 +89,13 @@ public class HighestUriQueuePrecedencePolicy extends BaseQueuePrecedencePolicy {
          */
         @Override
         public int getPrecedence() {
-            // base plus highest URI still in queue
-            Integer delta = (enqueuedCounts.size() > 0) ? enqueuedCounts.firstKey() : 0;
-            return super.getPrecedence() + delta;
+            // base plus highest URI still in queue
+            synchronized (enqueuedCounts) {
+                Integer delta = (enqueuedCounts.size() > 0)
+                        ? enqueuedCounts.firstKey()
+                        : 0;
+                return super.getPrecedence() + delta;
+            }
         }
 
         /*
@@ -98,20 +106,25 @@ public class HighestUriQueuePrecedencePolicy extends BaseQueuePrecedencePolicy {
             StringBuilder sb = new StringBuilder();
             sb.append(super.shortReportLegend());
             sb.append(":");
-            for(Integer p : enqueuedCounts.keySet()) {
-                sb.append(" p");
-                sb.append(p);
+            synchronized (enqueuedCounts) {
+                for (Integer p : enqueuedCounts.keySet()) {
+                    sb.append(" p");
+                    sb.append(p);
+                }
             }
             return sb.toString(); 
         }
 
         @Override
         public void shortReportLineTo(PrintWriter writer) {
-            boolean betwixt = false; 
-            for(Long count : enqueuedCounts.values()) {
-                if(betwixt) writer.print(" ");
-                writer.print(count);
-                betwixt = true;
+            boolean betwixt = false; 
+            synchronized (enqueuedCounts) {
+                for (Long count : enqueuedCounts.values()) {
+                    if (betwixt)
+                        writer.print(" ");
+                    writer.print(count);
+                    betwixt = true;
+                }
             }
         }