_Smilisca_ is an excellent example of an Autochthonous Middle American genus. As defined by Stuart (1950) the Autochthonous Middle American fauna originated from "hanging relicts" left in Central America by the ancestral fauna that moved into South America and differentiated there at a time when South America was isolated from North and Middle America. The genus _Smilisca_, as we define it, consists of six species, all of which occur in Central America. One species ranges northward to southern Texas, and one extends southward on the Pacific lowlands of South America to Ecuador. We consider the genus _Smilisca_ to be composed of rather generalized hylids. Consequently, an understanding of the systematics and zoogeography of the genus can be expected to be of aid in studying more specialized members of the family. Acknowledgments Examination of many of the specimens used in our study was possible only because of the cooperation of the curators of many systematic collections. For lending specimens or providing working space in their respective institutions we are grateful to Doris M. Cochran, Alice G. C. Grandison, Jean Guibe, Robert F. Inger, Guenther Peters, Gerald Raun, William J. Riemer, Jay M. Savage, Hobart M. Smith, Wilmer W. Tanner, Charles F. Walker, Ernest E. Williams, and Richard G. Zweifel. We are indebted to Charles J. Cole and Charles W. Myers for able assistance in the field. The cooperation of Martin H. Moynihan at Barro Colorado Island, Charles M. Keenan of Corozal, Canal Zone, and Robert Hunter of San Jose, Costa Rica, is gratefully acknowledged. Jay M. Savage turned over to us many Costa Rican specimens and aided greatly in our work in Costa Rica. James A. Peters helped us locate sites of collections in Ecuador and Coleman J. Goin provided a list of localities for the genus in Colombia. We especially thank Charles J. Cole for contributing the information on the chromosomes, and Robert R. Patterson for preparing osteological specimens. We thank M. J. Fouquette, Jr., who read the section on breeding calls and offered constructive criticism. Permits for collecting were generously provided by Ing. Rodolfo Hernandez Corzo in Mexico, Sr. Jorge A. Ibarra in Guatemala, and Ing. Milton Lopez in Costa Rica. This report was made possible by support from the National Science Foundation (Grants G-9827 and GB-1441) and the cooperation of the Museum of Natural History at the University of Kansas. Some of the field studies were carried out in Panama under the auspices of a grant from the National Institutes of Health (NIH GM-12020) in cooperation with the Gorgas Memorial Laboratory in Panama. Materials and Methods