A 12-month field study on sandflies was conducted from April 1992 to March 1993 at three locations in Baringo District, Rift Valley Province, Kenya. Study sites were selected based on their close proximity to the households of three patients who had had parasitologically confirmed visceral leishmaniasis in 1988, 1990 or 1991. Sandfly populations were highest during the two rainy seasons, April-June and November-December. A significant association was detected between the monthly abundance of sandflies and rainfall in the previous month. A significantly higher proportion of females than males was collected in light traps than on sticky paper traps set in termite mounds or animal burrows: approximately twice as many males as females were collected from the sticky paper traps whereas twice as many females as males were collected in light traps. Over 10% of the female sandflies collected from animal burrows and termite mounds were gravid, indicating that these flies were seeking oviposition sites at these locations. A total of 2231 sandflies was collected and five females were found to have flagellates in their midguts. The flagellates cultured from four Sergentomyia spp. were tentatively identified as Crithidia sp. by cellulose acetate electrophoresis (CAE). The promastigotes from the one Phlebotomus martini female found with an infected midgut were identified as Leishmania donovani by CAE.(ABSTRACT TRUNCATED AT 250 WORDS)