Minor ontogenetic changes in structure involve the shape of the snout, relative size of the eye, development of the tympanum, and amount of webbing in the hand. In recently metamorphosed young the snout is more rounded than in adults; the canthus and loreal concavity are not evident. Usually the tympanum is not differentiated in recently metamorphosed young, and the eye is proportionately large. The webbing in the feet is completely developed at metamorphosis, but young individuals have noticeably less webbing in the hand than do adults of the same species. Coloration Some of the most distinctive characters of the species of _Smilisca_ are color and pattern of the living frogs. Although many chromatic features are lost or subdued in preserved specimens, the patterns usually persist. _Metachrosis_ Change in color, well known in frogs, is common in hylids, especially in species having green dorsal surfaces (_Phyllomedusa_ is a notable exception). The non-green _Smilisca_ (_puma_, _sila_, and _sordida_) changes color, but this mostly is a change in intensity of color. In these species the markings usually are most distinct at night; frequently by day the frogs become pallid. The most striking examples of metachrosis in _Smilisca_ are found in the _baudini_ group, in which the dorsal ground-color changes from green to tan; correlated with the change in ground-color may be a corresponding change in the dorsal markings, but the dorsal markings may change to the opposite color. Chromosomes Chromosomes of all six species of _Smilisca_ were studied by means of the propriono-orcein squash technique described by Duellman and Cole (1965). Karyotype analysis was attempted for several species by means of intraperitoneal injections of colchicine, which affected the mitotic cells as desired, but the testes examined contained too few mitotic cells to allow accurate determination of karyotypes. Haploid (_n_) chromosome numbers were determined from cells in diakinesis, metaphase I, and metaphase II of meiosis. Diploid (2_n_) chromosome numbers were determined from cells in late prophase and metaphase of mitosis. Chromosome counts from as few as 23 meiotic cells of _S. phaeota_ and as many as 80 cells of _S. sordida_ reveal a constant haploid (_n_) number of 12; counts of chromosomes in one to five mitotic cells in all species, except _S. sila_, reveal that the diploid (2_n_) number is 24. NATURAL HISTORY Breeding Like most hylid frogs _Smilisca_ is most readily collected and observed when individuals congregate for breeding. _Time of Breeding_