The effect of N-nitrozo-N-methylbiuret (NMB) on various features of Didymocladium ternatum, an organism producing termatin was studied. It was found that the lethal effect of NMB depended on the exposure time, prolongation of which resulted in increased numbers of the plus variants, morphological mutations and reverses into the populations. Variants possessing 8 times higher activity as compared to the initial culture were obtained as a result of selection of active variants.