The effect of Fumonisin B1, a mycotoxin produced by Fusarium moniliforme on bacterial viruses P1 and Lambda, was investigated by the virus plaque assay. Fumonisin B1 inhibited the P1 viral multiplication in the concentration range from $100{mu}g$/ml to $400{mu}g$/ml. The inhibition was Fumonisin B1 concentration-dependent. Another bacterial virus Lambda multiplication was also inhibited by lower concentration of Fumonisin B1 ($10{mu}g$/ml~$50{mu}g$/ml). This inhibition was dependent on Fumonisin B1 and on virus-Fumonisin B1 reaction time. Sensitivity of bacteriophage Lambda to Fumonisin B1 was higher than that of P1 virus. Lambda vital DNA was treated in vitro with Fumonisin B1 at various concentration. Significant DNA fragmentation by Fumonisin 191 was observed in the agarose gel electrophoresis. Lambda viral DNA was partially digested even in the Fumonisin B1 $10{mu}g$ and the level of its fragmentation was dependent on Fumonisin B1 amount up to $30{mu}g$ per assay.