Because of the loss of productivity in industrial strains (as a consequence of genetic instability), the selection of spontaneous and induced mutants in Streptomyces might generate enhanced producers of bioactive compounds. In this work, a spontaneously high producing mutant of Streptomyces avermitilis, strain 267/2H, was isolated. This mutant produced 8.2 times more avermectin B-1 than the wild type and it was treated with methyl methanesulphonate (MMS) in order to obtain better avermectin producers. One mutant, strain IPT-85, produced about 16 times more avermectin than the wild-type strain ATCC 31267 and twice as much as the parental strain 267/2H. Reversion studies showed that avermectin production by the IPT-85 mutant was unstable and required constant selection to maintain high levels of avermectin B1 production. Upon a second MMS treatment of IPT-85, a new avermectin-aglycone-producing mutant, strain IPT 85-62, was isolated.