The small yellow tree frogs, _Hyla microcephala_ and its relatives,
are among the most frequently heard and commonly collected frogs in
the lowlands of southern Mexico and Central America. The similarities
in size, proportions, and coloration of the different species have
resulted not so much in a multiplicity of specific names, but in
differences of opinion on the application of existing names to the
various taxa. For example, the populations on the Atlantic lowlands
have been known by three names, two of which have been applied to
other taxa. Much of the confusion has been the result of previous
workers' unfamiliarity with the animals in life and unawareness of the
intraspecific geographic variation in the most widespread species.
Independently we undertook studies of these frogs in the field. The
second author worked on the interspecific relationships and isolating
mechanisms in Panama (Fouquette, 1960b) and later studied the species
in southern Mexico. As part of his survey of the hylids of Middle
America, the first author accumulated field and laboratory data on the
frogs throughout their ranges in Mexico and Central America. The
purpose of this report is to present our findings on the four species
of Middle American frogs that we place in the _Hyla microcephala_
group. In addition to conventional taxonomic characters, we have
utilized the features of the cranial osteology and have relied heavily
on the data obtained from an analysis of the mating calls.
Furthermore, we have included ecological and distributional data in
our synthesis of interspecific relationships.