The entomopathogenic nematodes Steinernema carpocapsae (Weiser) and S. glaseri (Steiner) are pathogenic to engorged adult, blacklegged ticks, Ixodes scapularis (Say), but not to unfed females, engorged nymphs, or engorged larvae. Nematodes apparently enter the tick through the genital pore, thus precluding infection of immature ticks. The timing of tick mortality, and overall mortality after 17 d, did not differ between infections by S. carpocapsae and S. glaseri. These nematodes typically do not complete their life cycles or produce infective juveniles in I. scapularis. However, both species successfully produced infective juveniles when the tick body was slit before nematode infection. Mortality of engorged I. scapularis females infected by S. carpocapsae was greater than uninfected controls, but did not vary significantly with nematode concentration (50-3,000 infective juveniles per 5-cm-diameter petri dish). The LC50 was 347.8 infective juveniles per petri dish (5 ticks per dish). Hatched egg masses of infected ticks weighed less than those of uninfected controls. Mortality of infected ticks was greatest between 20 and 30?C, and was lower at 15?C.