none The reproductive performance of B. hebetor females was adversely affected by the consumption of sub-lethal doses of vinblastine. While all oogenic cell types present in the gonads at the time of treatment displayed various degrees of fecundity and/or fertility depression, the transitional cells proved to be the least susceptible to vinblastine damage. Vitellogenically active oocytes were most sensitive to vinblastine. In these oocytes development was blocked at or prior to the terminal growth phase. Oviposited eggs which did arise from the exposed vitellogenic oocytes were few in number and characterized by aberrant morphology. Embryogenic effects were predominantly due to pre-blastoderm formation damage and most prnounced in oocytes exposed during the most advanced stages of gonadal development, late vitellogenesis through meiotic metaphase I. Reduced egg hatchability also occurred in exposed undifferentiated oogonial cells but the effect was less severe than that seen in the more mature oogenic cells. All observed effects could be accounted for by vinblastine's selective interference with microtubule-dependent processes. Fecundity effects were most closely associated with oocyte cortex and possibly follicular cell damage which prevented vitellogenic growth beyond the terminal growth phase. Fertility effects were caused by the inhibition of early embryonic karyokinesis with the most plausible target being mitotic spindle formation.