_Effect of temperature on calls._--The present data are insufficient to test statistically the correlation between temperature and variation within certain components of the calls in _Smilisca_, but even a crude graph shows some general correlations. The widest range of temperatures is associated with the recordings of _S. baudini_. Three individuals recorded at a temperature of 30 deg. C. at Tehuantepec, Oaxaca, had pulse rates of 180 pulses per second and fundamental frequencies of 160-180 cps., as compared with an individual recorded at a temperature of 17 deg. C., which had a pulse rate of 140 and a fundamental frequency of 135 cps. All individuals of _S. baudini_ recorded at higher temperatures had faster pulse rates and higher fundamental frequencies. Pulse rates differ in the other species in the genus but less strikingly (probably owing to narrower ranges of temperatures at which recordings were made). In five recordings of _S. sordida_ made at 20 deg. C. the pulse rate is 80-90, as compared with four recordings made at 25 deg. C. having pulse rates of 120-135. Thirteen recordings of _S. sila_ made at 17 deg. C. have pulse rates of 97-112 (average 105); one individual recorded at 26 deg. C. has 120 pulses per second. Seemingly no correlation exists between temperature and other characteristics of the calls, such as duration and rate of note-repetition. _The breeding call as an isolating mechanism._--Blair (1958), Bogert (1960), Duellman (1963a), Fouquette (1960), Johnson (1959), and others have provided evidence that the breeding calls of male hylids (and other anurans) serve as isolating mechanisms in sympatric species. In summarizing this discussion of the breeding calls of _Smilisca_ we want to point out what seem to be important differences in the calls that may prevent interspecific hybridization in sympatric species of _Smilisca_. The genus is readily divided into two species-groups on morphological characters; this division is supported by the breeding calls. In the species of the _baudini_ group the calls are unmodulated and lack secondary notes. In the _sordida_ group the calls either have secondary notes or are modulated. _Smilisca baudini_ occurs sympatrically with _S. cyanosticta_ and _S. phaeota_; where they occur together, both species sometimes breed in like places at the same time. We are not aware of these species breeding synchronously at exactly the same site, although _S. baudini_ and _S. cyanosticta_ were calling on the same nights and less than 100 meters apart in Oaxaca in June, 1964. Regardless of their respective breeding habits, sympatric species have calls that differ notably. Except for the higher fundamental and dominant frequencies, the calls of _S. cyanosticta_ and _S. phaeota_ closely resemble one another, but the calls of both species differ markedly from that of _S. baudini_. The geographic ranges of _S. cyanosticta_ and _S. phaeota_ are widely separated.