During studies on leishmaniasis transmission in Panama, wild-caught female Phlebotomus sandflies (Diptera; Psychodidae) were dissected and examined for leptomonads. A total of 198 of 3112 females of 6 man-biting species collected from January through July 1961, were infected. Pure cultures were obtained 44 times. One strain produced lesions in Syrian hamsters like those produced by Panamanian human strains of Leishmania. In both infected wild-caught females and laboratory-reared females infected by feeding on animals with lesions produced by Panamanian human strains, leptomonads were always present in the hindgut. A Peruvian espundia strain gave the same hindgut infection pattern, while a Guatemalan strain invariably produced infections in the midgut, with few or no flagellates in the hindgut.