Both sexes of the milkweed beetle, Tetraopes tetraophthalmus (Forster), mate many times over the course of a 6-week adult life span. In laboratory experiments, the lifetime fertility of females allowed to mate several times in a 24-h period but never again was the same as that of females allowed continual access to males. Females mating only once showed reduced lifetime fertility. In the laboratory the most recent male to mate with a female fertilizes about 72% of the eggs deposited in the next egg clutch. Mixing of the sperm of multiple fathers in the fertilization of a given clutch of eggs was also demonstrated in the field.