Size variation occurs in natural populations of the black blow fly, Phormia regina (Meigen); head width varied from 2.25 and 1.89 mm to 3.68 and 3.72 mm for males and females, respectively. Using these values as a guide, small flies were produced in the laboratory by early removal of larvae from the rearing medium. Mating experiments, pairing flies of various size categories, showed that when small males mated with large females the percentage of successfully inseminated females was significantly reduced compared with the other combinations (42% versus 73–89%). Previous work led us to suspect that small males might have small aedeagi and therefore have a problem in successful sperm transfer when they mate with large females. Measurement of aedeagus size and head width showed that in wild flies the size of the aedeagus is positively correlated with body size.