The pesticide DDT was examined for possible mutagenicity in mice, D. melanogaster and N. crassa. Through the use of the dominant-lethal assay it was found that acute oral doses of DDT (2 x 150 mg/kg body weight) in male mice induced dominant lethal mutations in early spermatid and spermato-cyte stages. Chronic oral doses of DDT (2 x 100 mg/kg body weight per week for 10 weeks) in male mice caused a persistent increase in the number of dominant lethal mutations. Histological sections showed that chronic treatment of mice with DDT caused changes in seminiferous tubule morphology and degeneration of B-type spermatogonia. Acute treatment of mice with DDT caused an increase in spermatocyte chromosome breakage, stickiness and precocious separation of the X and Y bivalent.