Abstract In the search for vectors of leishmaniasis in Panama, over 5000 wild-caught Phlebotomus females were dissected and examined for leptomonad flagellates. Natural infections were found in 416 out of a total of 4861 females of six of the seven common man-biting species ( gomezi, panamensis, sanguinarius, shannoni, trapidoi, and ylephiletor). A single female longipalpis was also infected. None of 365 females of 13 other species which never or only occasionally bite man was infected. A total of 262 males, including 242 of common man-biting species, was negative. Leptomonad infections were found in sandflies from each of five endemic areas and in two others where human cases have not been reported. The over-all infection rate of the common man-biters was 8.5%, with rates varying from 1.9% for panamensis to 15.4% for trapidoi. During the rainy season, June through December, the over-all rate (10.6%) was much higher than during the dry season (4.1%). The source of these infections in unknown. However, the vertical distribution of species in simultaneous ground-level and tree-platform catches, together with differential infection rates, suggest that the reservoirs may be found among arboreal animals. Leptomonads were always present in the hindgut of infected females. They also occurred occasionally in the posterior part of the midgut (stomach), rarely in the anterior part (cardia), and only twice were they found in the foregut (esophagus and pharynx, respectively). In over 20% of the infections leptomonads occurred in the Malpighian tubules. In culture the morphology of leptomonads from the natural infections is consistent with that of Leishmania. Lesions produced in hamsters by two of the sandfly strains are indistinguishable from those produced by Panamanian human strains. In the sandfly gut the growth pattern and morphology of the leptomonads in natural infections are similar to those of sandflies fed on hamster lesions produced by both Panamanian human strains and a wild-caught sandfly strain.