Breeding populations of phlebotomine sandflies (Diptera, Psychodidae) on the floor of a rain forest in Panama were studied with soil emergence traps. Supporting studies were made of physiographic and hydrologic features of the breeding habitat. The pattern of distribution of the forest litter and the composition, texture and stability of local deposits are largely determined by physiographic and hydrologic features which are operative in soil transport. The forest litter tends to be removed from steep slopes and valleys and to cumulate on gentle slopes and prominences. On hilltops the forest litter tends to occur in stable, residual deposits, while on hillsides and stream banks relatively unstable, alluvial deposits are more common. Breeding populations of Lutzomyia pessoana (Barretto), L. panamensis (Shannon), L. gomezi (Nitzulescu), and L. insolita (Fairchild and Hertig) are greater in hilltop regions, while those of L. trapidoi (Fairchild and Hertig) are greater in hillside and streamside regions. No comparable effect was noted in the case of L. rorotaensis (Floch and Abonnenc). No effects of physiographic aspect or of non-destructive inundation on breeding populations of sandflies were detected.