The susceptibility of promastigote of Leishmania major to Nystatin in vitro was examined. L. major (MHOM/PK/88/DESTO) promastigote were cultured in medium 199 supplemented with 10% heat inactivated foetal bovine serum and 2% urine. The growth of the promastigote was monitored in the absence and presence of the experimental compound (Nystatin) for upto 5 days post-inoculation. The EC50 value (the concentration of drug necessary to inhibit the growth rate of cells to 50% of the control value) obtained for Nystatin against the promastigote of L. major was less than 9.76 iu ml. Certain polyene compounds like Amphotericin-B and Nystatin (mycostatin) are familiar for their fungicidal activity. Amphotericin-B is used since long as antileishmanial drug as well. Results obtained suggest that Nystatin has a very good anti leishmanial activity in vitro. The mode of action proposed for this drug is same as for Amphotericin-B as both of these polyene compounds interact with the various sterols present on the surface of the parasite, thus unusual gaps and pores are formed on the surface that results in the leakage of the ions. This leakage finally leads to the destruction of the parasite.