Aphelinus asychis Walker (Hymenoptera: Aphelinidae) is a solitary koinobiont endoparasitoid, which parasitizes the green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae). Host discrimination is the ability of a parasitoid to distinguish between unparasitized and parasitized hosts, so parasitoids can avoid wasting eggs and time. We determined the influence of host density (5, 10, 15, 20, and 25 aphids per experienced parasitoid during a 24-h period) on egg distribution, and tested the effects of different time intervals (0, 24, 48, and 72 h) between successive ovipositions on host discrimination between unparasitized and self- or conspecific-parasitized aphids. The results demonstrated that superparasitism decreased with increasing host density. Egg distributions were nonrandom at all host densities, and the observed number of self-superparasitized hosts was lower than expected at all host densities. At 0-h interval, most parasitoids used only antennation to examine and reject parasitized aphids, which might be associated with an external marker. At longer time intervals (≥24 h), A. asychis most often used antennation followed by ovipositor insertion to examine and reject previously parasitized aphids, probably responding to an internal, from chemical substances injected during the first oviposition or changes in host quality associated with parasitism. The results showed that host discrimination in A. asychis was mediated by two consecutive cues, the first external and the second internal, with the relevancy of each depending on the time interval between two ovipositions. Consequently, A. asychis was able to discriminate between unparasitized and self- or conspecific-parasitized M. persicae, but always engaged in some superparasitism.