Semen characteristics of captive American Kestrels (Falco sparverius) fed 8 ppm mirex or 33 ppm Aroclor^® 1254, singly or in combination, were compared to a control group. Aroclor^® 1254 produced a decline in sperm concentration but no compensatory increase in semen volume, resulting in a 22–27% decrease in sperm numbers per ejaculate. Mirex produced a marked decline in sperm concentration with a slight compensatory increase in semen volume resulting in a 70% decrease in sperm numbers. The combination of Aroclor^® 1254 and mirex, at the same concentrations as fed singly, decreased sperm concentration to the same level as the Aroclor^® alone with a 73% increase in semen volume, resulting in sperm numbers which did not differ from those of the controls. No effect on sperm motility was observed for any contaminant. Temporal patterns in semen characteristics differed. There was a highly significant interaction effect between Aroclor^® 1254 and mirex. The excretion of mirex was increased in the presence of Aroclor^® 1254, and the combination diet reduced the relative concentration of Aroclor^® in the testes. The testicular mass of the PCBs + mirex group was increased. The results suggest that migratory flesheating birds feeding on a PCB- or mirex-contaminated food chain could consume enough toxicant to alter their semen quality in that breeding season, which, when coupled with altered courtship, could reduce the fertility of eggs and the reproductive fitness of the individual.