Mating-call structure was used to determine the nature and extent of an interaction between the two morphologically indistinguishable taxa, C. laevis and C. victoriana, where their geographic ranges contact in south-western Victoria. Intermediacy of mating-call structure was detected in a zone about 115 km long and less than 2 km wide. Acoustic analysis of call samples from six localities indicated that hybrids, backcross progeny, and one or both parental types were present in this zone. Results of reciprocal artificial hybridization tests between allopatric individuals of the two taxa showed that there was a high level of interfertility in both combinations, but with slightly reduced survival to metamorphosis. Presumed backcrosses using males from the hybrid zone resulted in a higher level of survival to metamorphosis. The zone does not coincide with any conspicuous ecotone and presumably represents a complex balance point with various environmental factors contributing to a greater or lesser extent in different parts of the contact. In spite of this lack of reproductive isolation, the narrowness of the zone and the presumed antiquity of the contact demonstrate that the taxa will continue to maintain their distinctness, and should retain their specific status.