The bruchid beetleStator vachelliaeBottimer will sometimes oviposit on seeds of the nonhostParkinsonia aculeata(Leguminosae = Fabaceae), but most larvae do not survive. Of 1,118 eggs onParkinsoniaseeds, 86% occurred in a narrow zone where seeds ofParkinsoniaand the normal leguminous hostAcacia flexuosawere mixed on the ground beneath overlapping canopies. A field experiment indicated that females use nonseed cues associated withAcacia(pod valves and cow and horse dung) to locateAcaciaseeds, and when seeds of both hosts are encountered, females oviposit equally on seeds of both plants. Other factors that were not controlled in the experiments (amount of daylight, temperature, humidity) may also affect discrimination between seeds of different hosts. Survivorship onParkinsoniawas<0.1% in the field, whereas onAcaciait was 15%. Most (53%) mortality onParkinsoniawas attributable to inability of larvae to penetrate seed coats. Enemy-free space was available onParkinsonia:only 3% of bruchid eggs on its seeds were parasitized, but on the normal hostAcacia,24% were parasitized. We suggest three steps for S.vachelliaeto includeParkinsoniain its host range or for a host shift: (1) oviposition on the potential host in proximity to cues associated with the normal host, which does not appear to require a genetic change in search or oviposition behavior; (2) larvae of S.vachelliaemust evolve the ability to complete development in the new host; and (3) the adult beetles must be able to track the new host. Only step 1 has occurred in this system. The results support previous suggestions that the initial colonization of new hosts may be behavioral.