_Remarks._--_Hyla microcephala underwoodi_ has had a confused nomenclatural history. The taxon was first named _Hyla microcephala_ by Boulenger (1898); this name was preoccupied by _Hyla microcephala_ Cope (1886). Cole and Barbour (1906) and Kellogg (1932) used the name _Hyla phlebodes_ Stejneger (1906) for specimens of this frog from México. Dunn (1931, 1933, 1934) applied the name _Hyla underwoodi_ to Panamanian specimens that we identify as _Hyla phlebodes_. Smith (1951) named _Hyla microcephala martini_ from southern México and Guatemala and considered the northern populations to represent a subspecies distinct from the Costa Rican _Hyla microcephala underwoodi_, despite the fact the Stuart (1935:39) stated that comparisons of specimens from El Petén, Guatemala, with the holotype of _Hyla underwoodi_ showed only trivial differences. Much of the confusion regarding the name _Hyla underwoodi_ stems from the illustration given by Boulenger (1898:pl. 39, fig. 3) and reproduced by Taylor (1952:892), which shows a frog having a unicolor dorsum, dorsolateral white lines, and dark flanks. This pattern is in marked contrast to the pattern seen in most preserved specimens, which have the dorsum variously marked by dark brown lines or irregular marks. Smith (1951:185), in his description of _Hyla microcephala martini_ from southern México, considered _H. underwoodi_ to be a subspecies of _H. microcephala_ that lacked dorsal dark markings. Data accumulated in 1961 through field studies by the senior author at the type locality, Bebedero, and other localities in Guanacaste and Puntarenas provinces in Costa Rica provide a reasonable explanation of the differences in color pattern. As noted in the preceding description of this subspecies, at night the dorsal markings are not evident in many living individuals, whereas by day the dorsal markings are prominent. Most collectors prepare their specimens by day; consequently the majority of specimens have a pronounced dorsal pattern. Of the frogs collected in Costa Rica in 1961, some specimens were preserved at night; others from the same series were preserved by day. The differences are striking. In those preserved at night, dorsal markings are faint, if present at all. Some specimens closely match the figure given by Boulenger (1898).