Of 229 residents of a Panamanian village where both Trypanosoma cruzi and Trypanosoma rangeli are endemic, 52% had antibody to one or both species by the enzymelinked immunosorbent assay (ELISA), 26% were positive by complement fixation, 17% were positive by direct agglutination, and 32% were positive on the basis of clinical impression. Although the sensitivity of ELISA makes it the procedure of choice for sero-epidemiologic studies, there does appear to be some serologic cross-reactivity between the two species. Sera from 55 Panamanians and 33 Bolivians were reactive with antigens of T. cruzi and T. rangeli, thus making definitive diagnosis by serologic methods impossible. Although the presence of antibodies to both species suggests serologic cross-reactivity, the possibility of dual infection must be considered also. Fifty-four Panamanians and 20 Bolivians had antibody to only one species.