diff --git a/guix/monads.scm b/guix/monads.scm
index 6ae616aca..c9c5da3bb 100644
--- a/guix/monads.scm
+++ b/guix/monads.scm
@@ -20,6 +20,7 @@
   #:use-module ((system syntax)
                 #:select (syntax-local-binding))
   #:use-module (ice-9 match)
+  #:use-module (ice-9 threads)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-9)
   #:use-module (srfi srfi-26)
@@ -117,6 +118,7 @@
   ;; the syntax object of the parameter over which it is templated, and (2)
   ;; the syntax of its body.
   (define-once %templates (make-hash-table))
+  (define-once %template-lock (make-mutex))
 
   (define (register-template! name param body)
     (hash-set! %templates name (cons param body)))
@@ -139,8 +141,9 @@ template instances."
       (syntax-source s))
 
     (define current-info-port
-      ;; Port for debugging info.
-      (const (%make-void-port "w")))
+      ;; Port for debugging info.  Return a fresh port at each call to make
+      ;; sure we're thread-safe.
+      (lambda () (%make-void-port "w")))
 
     (define location-string
       (format #f "~a:~a:~a"
@@ -204,12 +207,14 @@ template instances."
        ;; Search for an instance of template NAME for this ACTUAL parameter.
        ;; On success, expand to the identifier of the instance; otherwise
        ;; expand to #f.
-       (any (matching-instance? #'name #'actual) %template-instances))
+       (with-mutex %template-lock
+         (any (matching-instance? #'name #'actual) %template-instances)))
       ((_ exists? name actual)
        ;; Likewise, but return a Boolean.
        (let ((result (->bool
-                      (any (matching-instance? #'name #'actual)
-                           %template-instances))))
+                      (with-mutex %template-lock
+                        (any (matching-instance? #'name #'actual)
+                             %template-instances)))))
          (unless result
            (format (current-warning-port)
                    "~a: warning: no specialization of template '~a' for '~a'~%"
@@ -220,8 +225,9 @@ template instances."
        ;; Expand to the definitions of all the existing templates
        ;; specialized for ACTUAL.
        #`(begin
-           #,@(hash-map->list (cut instance-definition <> <> #'actual)
-                              %templates))))))
+           #,@(with-mutex %template-lock
+                (hash-map->list (cut instance-definition <> <> #'actual)
+                                %templates)))))))
 
 (define-syntax define-template
   (lambda (s)