Boron compounds are increasingly recognized as important industrial pollutants. We examined whether elevated concentrations of boron would exert toxic effects on different life cycle stages of the rodent gastrointestinal nematode Heligmosomoides bakeri and established toxicity in feeding and nonfeeding stages. All stages except eggs responded in a dose-dependent manner to boron. Reduced motility of first-stage larvae immediately after hatching and of the larvae exposed to boron only at the third larval stage were observed. Moreover, exposure of third-stage larvae to high concentrations of boron reduced their infectivity to mice. Survival of first- and second-stage larvae and of fourth-stage larvae and adults cultured in vitro in boron was impaired. Interestingly, adult males were more sensitive to boron than adult females. Finally, per capita fecundity of females cultured in vitro decreased with increasing boron concentration. Together, these results show that high boron concentrations are harmful to free-living stages and to parasitic stages in vitro and support our hypothesis that nonfeeding stages (eggs and third-stage larvae) are more resistant to boron than feeding stages (first-, second-, and fourth-stage larvae and adult worms).