Body size of wild mating males and females of the cactophilic species Drosophila buzzatii was larger and tended to be less variable than that of randomly sampled flies. The intensity of sexual selection was estimated to be 0·34 in males and 0-16 in females (average 0·25). Coefficients of rank correlation for the body size of mating pairs are not statistically different from 0, pointing out that no significant assortment for size occurs in our sample. The results can be interpreted as due to the vigour or general activity levels of larger flies which are more likely to encounter suitable mates than smaller ones, although differences in size could exist among age-classes. © 1988, The Genetical Society of Great Britain.