From mating of Xc2/BsYy+ males treated or not with 2.5 mM DMN (dimethylnitrosamine) with repair-deficient mei-9a females or with ordinary females, induced frequencies of observed (recovered) chromosome loss were 3.69% and 0.65% and inferred (non-recovered) ring-X loss based on shifts in sex ratio (male male/female female) was 47.9% and 9.4%, respectively. Results indicate that the mei-9a female is a strong potentiator of DMN-induced chromosome damage in sperm and suggest that DMN-induced chromosome lesions are produced in substantially higher frequency in treated sperm than deducible after crosses with repair-efficient females.