diff --git a/CHANGES b/CHANGES
index 75b5a0aac9..ca1b2d701e 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 aa11ade593..c8a06b4610 100644
--- a/lib/dns/include/dns/rdata.h
+++ b/lib/dns/include/dns/rdata.h
@@ -502,7 +502,7 @@ 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 645136256d..982131efeb 100644
--- a/lib/dns/rdata.c
+++ b/lib/dns/rdata.c
@@ -100,7 +100,7 @@
 #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
 
@@ -1222,7 +1222,7 @@ 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;