We examined the influence of differential selection by natural enemies on the number of chambers in the galls of two closely related gall midges: Asteralobia sasakii and A. soyogo. To evaluate the selection pressure imposed by parasitoid wasps, we investigated the number of larval chambers (as represented by the number of larvae) and the rate of parasitism in each gall. Galls with smaller numbers of either species of cecidomyiid larvae were attacked more frequently by ectoparasitoids. In A. sasakii galls, the overall rate of attack by ectoparasitoids was higher than in A. soyogo galls. especially in galls with many larvae. Midge larvae of both species that hatched from larger egg clutches were attacked more frequently by endoparasitoids. In A. sasakii, the overall rate of attack by endoparasitoids was lower than in A. soyogo, especially in large clusters of larvae. Consequently, the final survival rate of A. sasakii peaked at higher larval numbers (14.5 per gall in 2000-01 and 10.1 per gall in 2001-022) than for A. soyogo (5.5 per gall in 2000-01 and 6.4 per gall in 2001-02). The mean number of larvae per gall was also higher in A. sasakii than in A. soyogo. Differences in the selection pressures exerted by natural enemies led to differences in the numbers of chambers in galls of the two different midge species and, in turn, different oviposition behaviours of midge females, reflecting variations in counter-strategies against parasitoids. Evolution; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Land zones; Palaearctic region; Eurasia; Asia Asteralobia sasakii; Asteralobia soyogo (Cecidomyiidae ); Natural selection; Hymenopteran parasites; Plant hosts; Ilex; Gall chamber number; Japan; Honshu; Chiba; Gall chamber number on plant host, influence of hymenopteran parasite attack rate Bracon tamabae (Braconidae ); Gastrancistrus; Mesopolobus (Pteromalidae ); Torymus (Torymidae ); Inostemma (Platygasteridae ); Dipteran hosts; Asteralobia; Attack rate, effect on gall chamber number on plant host; Japan; Honshu; Chiba; Attack rate on dipteran host in plants, effect on gall chamber number none