In many parts of the world, the larvae of the cabbage white butterflies, Pieris rapae and P. brassicae, are considered to be major pests in several economically important brassicaceous crops including various cultivars of cabbage and mustard. Thus far, biological control of these pests has focused on parasitoids including species in the genus Cotesia. We examined interactions between the solitary ichneumonid parasitoid, Hyposoter ebeninus, developing in 1st to 3rd (L1–L3) larval instars of both P. rapae and P. brassicae. H. ebeninus is common in central and southern Europe, but has thus far received little attention as a possible biological control agent of cabbage butterflies. Larvae of both pierids continued to grow after they were parasitized, and development was only arrested some 5–7 days later. Caterpillars parasitized in the third instar grew significantly larger than larvae parasitized as L1 or L2. Adult parasitoid body mass was inversely correlated with host instar parasitized, and female wasps were significantly larger than male wasps. Egg-to-adult development time in H. ebeninus did not vary between the two hosts, but the parasitoid exhibited protandry. Parasitoid survival was generally higher in larvae of P. rapae than in larvae of P. brassicae, but varied with instar in the two hosts. Our study shows that H. ebeninus has promise as a biological control agent, particularly against its more suitable host, P. rapae.