Malathion is an organophosphorus pesticide that acts as an acetylcholinesterase inhibitor and induces DNA damage. It was widely used in Arica, Chile, to eliminate the fruit fy Ceratitis capitata. The aim of this study was to analyze the effects of malathion on the cellularity of the seminiferous epithelium and cell proliferation in the mouse testis. Thirty adults mice of the CF1 strain were divided into control group and fve experimental groups (n=5 each). The experimental groups were treated with single intraperitoneal (i.p.) doses of malathion, 240mg·kg-1 (1/12 LD50) and analyzed on days 1; 8.6; 17.2; 34.4 and 43 post treatment. Subsequently, the right testis was processed for histology and the left testicle was incubated for 1h in BrdU for immunocytochemistry. The results show that at the applied dose, malathion alters the progression of CF1 mice spermatogenesis, causing a decrease in DNA synthesis and cell proliferation. We conclude that adult CF1 mice treated with malathion expressed from acute effects in the early days of analysis (1 and 8.6 days) to a chronic effect at 17.2, 34.4 and 43 post treatment days, with higher rates of tubular plugging and increased vacuolization in the germinal epithelium of the testis. The use, handling and storage of agropesticides requires stricter standards and protocols, not to alter the environment and public health.