_Variation._--The snout-vent length in seven adult males is 47.2-50.0 mm., and in five females, 45.1-52.5 mm. In neither sex do the average proportions differ noticeably from those of the holotype, except that the tympanum is relatively larger in females. The ratio of the diameter of the tympanum to that of the eye is 0.363-0.468 (mean 0.438) in males and 0.500-0.545 (mean 0.516) in females. The average number of prevomerine teeth in males is 7.9, in females 8.4. In life dorsal coloration varied from pale green to olive-green with darker green or black flecks or reticulations, or pinkish tan to brown with dark brown flecks or reticulations. Some preserved specimens have relatively few dark flecks, whereas in most specimens the dorsum is heavily marked. All specimens have some white markings above the anus and on the posterodorsal surfaces of the thighs, but in some individuals the white flecks are expanded and interconnected forming an irregular white line. Juveniles have a notably different coloration in life. The dorsum is uniform pale green; the anterior and posterior surfaces of the thighs, fingers, first three toes, and webbing are deep yellow. The anal stripe is creamy white, and the flanks are pale gray with black flecks. The upper lip, supratympanic fold, and canthal stripe are a bronze color. The belly is pale yellow with a silvery cast on the throat. Juveniles having snout-vent lengths from 24.5 to 36.6 mm. are so colored in life, and uniform dark bluish gray dorsally in preservative. _Comparisons._--The absence of a quadratojugal and the presence of a greatly enlarged, non-projecting prepollex place _Hyla siopela_ in the _Hyla bistincta_ group (see Duellman, 1964, and Adler, 1965). The presence of a rostral keel separates _Hyla siopela_ from other members of the _Hyla bistincta_ group, which is composed of two species having long anal sheaths (_bistincta_ and _pentheter_), two small species having axillary membranes and lacking nuptial excrescences in breeding males (_charadricola_ and _chryses_), and three species (_crassa_, _pachyderma_, and _robertsorum_) having short heads, round snouts, short anal sheaths, and nuptial excrescences in breeding males. _Hyla siopela_ differs from the last three species in the shape of the snout and from each in certain structural features; _H. crassa_ has fully webbed feet; _H. pachyderma_ has large nuptial spines, and _H. robertsorum_ has more webbing and a shorter tarsal fold. Furthermore, the venter in _H. robertsorum_ is brown with creamy white flecks. In structure and coloration _H. arborescandens_ resembles _siopela_, but the former is smaller, and males of _arborescandens_ have vocal slits.