Hymenopteran insects, including parasitoid wasps, usually have a haplo-diploid sex-determination system. This enables an ovipositing female to choose the sex of her egg by controlling sperm access to the egg (Flanders, 1956). The potential for sex ratio control makes hymenopteran insects ideal experimental organisms for testing sex ratio theory (Charnov, 1982; Godfray, 1994). Although there are many sex ratio studies in parasitoid wasps, the data are based on sex ratio at wasp emergence. However, if differential mortality between immature sexes occurs, the sex ratio at wasp emergence will differ from that at oviposition, i.e., the sex ratio determined by the female wasps. In fact, in several parasitoid wasps, unsuitability of hosts and shortage of food are suggested to result in higher mortality of female larva than male larva (King, 1987). Because most sex ratio theories deal with sex ratio at oviposition, any studies using sex ratio at wasp emergence can be incorrect to test sex ratio theories, if different sex mortality occurs. Therefore, effects of differential sex mortality need to be clarified in sex ratio studies. To date, the effects of differential mortality on secondary sex ratio have been estimated by: 1) comparing mortality of offspring from mated females with that from unmated females (Sandlan, 1979); 2) egg transfer experiments between small and large hosts (Heinz & Parrella, 1990); 3) using a predictive model of primary sex ratio (Wellings et al., 1986); and 4) using differences in female behaviour as an index for predicting primary sex ratio (Suzuki et al., 1984; van Dijken & Waage, 1987; Ueno, 1995). These methods, however, are not suitable for determining exact sex ratio and are not always applicable. Since most Hymenoptera are haplo-diploid, cytological methods for observing chromosomes at the egg stage are available to determine exact primary sex ratio and are applicable to most parasitoid wasps. However, little research using cytological methods to determine primary sex ratio has been done (van Dijken et al., 1989, 1993). In this paper, we apply a cytological method to determine primary sex ratios in two idiobiont parasitoid wasps Pimpla nipponica Uchida and Itoplectis naranyae Ashmead (Hymenoptera: Ichneumonidae). These polyphagous parasitoids attack a variety of host species which differ in size (Ueno & Tanaka, 1994). In such a case, sex ratio control in response to host size could evolve (Charnov, 1982). Here we test whether these parasitoids change the sex ratio in response to host size and whether host size is responsible for differential mortality between immature sexes.