Size variation of flesfly,Wohlfahrtia nuba was produced in the laboratory by rearing larvae on different weights of meat. Mating experiments,pairing flies of various sizes showed that when small maless mated with large femles the percentage of successfuly inseminatd females was significantly reduced compared with large males.Measurments of aedeagus size and head width showed that the size of aedeagus is positively correlated with body size .The large maleswerw nearly twice as effective as small males at switching off females receptivity after numerous,successive matings when tested 24h after their initial mating.Beyond five matings small males were unable to switch off receptivity in more than 50% of the females.Whereas,large males still rendered over than 80%of the females unreceptive.On the other hand,the lag time increased an approximately twofold with small males compared to large males in successive matings.