Ixodes scapularis Say adults, nymphs, and larvae were treated with the entomopathogenic fungus Metarhizium anisopliae in a combination of field and laboratory experiments to assess sublethal effects of the fungus on I. scapularis fecundity and body mass. Postengorgement and egg mass weights were 33 and 50% lower, respectively, in adult females treated with M. ansiopliae in the field before engorging on laboratory rabbits. M. anisopliae did not significantly reduce egg mass weight, conversion efficiency, or oviposition period in I. scapularis females treated with the fungus after engorging on white-tailed deer, although only 33% of treated females oviposited. Engorged nymphs and larvae treated with M. anisopliae converted significantly lower percentages of their engorged weight to their molted adult and nymphal weights. This study indicates that M. anisopliae reduces fitness (fecundity and body mass) in all active I. scapularis stages and indicates that its impact as a biocontrol agent might be higher than that suggested by direct mortality alone.