would not be effective as an isolating mechanism. Secondly, since the range of variation of pulse repetition rate in deep sympatric H. verreauxidid not overlap that of allopatric H. verreauxi (a difference comparable with that seen in the heterospecific sympatric populations), premating reproductive isolation of these populations was predicted.Although differences in mating calls have been shown to operate as important premating isolating mechanisms in other species of hylid frogs (Blair and Little john, 1960;Littlejohn, 1960Littlejohn, , 1961Littlejohn, Fouquette, and Johnson, 1960;Littlejohn and Michaud, 1959;Michaud, 1962;Snyder and Jameson, 1965), direct experimental evidence was lacking in the H. ewingi complex. Accordingly, a program of call discrimination trials was arranged in order to test the assumption that the mating call differences were operating as premating isolating mechanisms. Since the calls of shallow sympatric H. ewingi are similar to those of adjacent allopatric H. ewingi, the predictions about the degree of premating isolation present between the two allopatric populations could also be tested. For practical reasons it was possible to test female H. ewingi and H. uerreauxi from only one area-western sympatry. Even so, the results of the trials are convincing, allowing some conclusions and further predictions to be made.