In 1975-1978 and in 1985-1988 studies of species composition and number of Phlebotomus, R. opimus infestation with L. major, L. turanica and L. gerbilli as well as epidemic activity of the natural foci were performed in the Karshi steppe in Uzbekistan. Typical areas have been compared in the desert, oasis and desert land irrigated for cotton growing. A correlation has been established between the epidemic activity of the natural focus and the nature of epizootic development in R. opimus and species composition of vectors. On desert territories epizootic transmission throughout the whole season develops with the domination of one leishmania species (L. turanica) which is not pathogenic for human beings. Paraphlebotomus are the main vectors transmitting leishmania. In oases epizootic process assumes a two-phase course, with accumulation by the middle of the season L. major species pathogenic for human beings. It is transmitted by P. papatasi. With desert irrigation, the former Phlebotomus species is transformed into the latter one, with the predominance of P. papatasi. Epizootics acquire a stable two-phase nature.