This study examines the influence of the size of the host patch (egg mass size) on the sex ratio produced by the scelionid egg parasitoid Trissolcus basalis (Wollaston) under laboratory conditions. The influence of this factor on the sequence in which male and female eggs are laid also is analyzed. Our detailed study shows that the size of the host patch offered to females strongly influenced both the overall sex ratio they produced, and the organization of the sequence in which male and female eggs are laid. Females adjusted their sex ratio with host number per host egg mass in qualitative agreement with the predictions of the local mate competition model and independently of oviposition sequence. Variations in the sex allocation sequences produced by T. basalis do not seem to depend on the ability of female wasps to assess the number of hosts per patch before oviposition. Females start egg-laying sequences by laying male eggs early in an oviposition bout in accordance with the males 1st strategy. As long as the sequences are completed, the pattern of sex allocation changes as an indirect modification in response to variation in the size of the egg mass. Such a modification in sex-sequence patterns can be considered to be an adaptive mechanism leading the females to adjust their sex ratio to different environmental conditions.