diff --git a/CHANGES b/CHANGES
index a6c65eca9f..3333080a61 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,6 @@
+4972.	[func]		Declare the 'rdata' argument for dns_rdata_tostruct()
+			to be const. [GL #341]
+
 4971.	[bug]		dnssec-signzone and dnssec-verify did not treat records
 			below a DNAME as out-of-zone data. [GL #298]
 
diff --git a/lib/dns/include/dns/rdata.h b/lib/dns/include/dns/rdata.h
index 969e5fa7b9..a487428fb4 100644
--- a/lib/dns/include/dns/rdata.h
+++ b/lib/dns/include/dns/rdata.h
@@ -501,7 +501,7 @@ dns_rdata_fromstruct(dns_rdata_t *rdata, dns_rdataclass_t rdclass,
  */
 
 isc_result_t
-dns_rdata_tostruct(dns_rdata_t *rdata, void *target, isc_mem_t *mctx);
+dns_rdata_tostruct(const dns_rdata_t *rdata, void *target, isc_mem_t *mctx);
 /*%<
  * Convert an rdata into its C structure representation.
  *
diff --git a/lib/dns/rdata.c b/lib/dns/rdata.c
index 0594bc2f01..6f9951709e 100644
--- a/lib/dns/rdata.c
+++ b/lib/dns/rdata.c
@@ -95,7 +95,7 @@
 #define ARGS_FROMSTRUCT	int rdclass, dns_rdatatype_t type, \
 			void *source, isc_buffer_t *target
 
-#define ARGS_TOSTRUCT	dns_rdata_t *rdata, void *target, isc_mem_t *mctx
+#define ARGS_TOSTRUCT	const dns_rdata_t *rdata, void *target, isc_mem_t *mctx
 
 #define ARGS_FREESTRUCT void *source
 
@@ -1185,7 +1185,7 @@ dns_rdata_fromstruct(dns_rdata_t *rdata, dns_rdataclass_t rdclass,
 }
 
 isc_result_t
-dns_rdata_tostruct(dns_rdata_t *rdata, void *target, isc_mem_t *mctx) {
+dns_rdata_tostruct(const dns_rdata_t *rdata, void *target, isc_mem_t *mctx) {
 	isc_result_t result = ISC_R_NOTIMPLEMENTED;
 	isc_boolean_t use_default = ISC_FALSE;