The minute chalcid wasp, Ixodiphagus hookeri, is known as a parasitoid of several ixodid ticks including the blacklegged tick, Ixodes scapularis. We determined quantitatively the prevalence and seasonal activity of the wasp in its tick host, I. scapularis, on Prudence Island, Rhode Island, USA. From May through August in 1988 and 1989, the seasonal peak of nymphal ticks was reached in June. The highest prevalence of wasp parasitism in tick hosts occurred in May, approximately one month earlier than the seasonal peak of the tick. In addition, we observed that wasp parasitism was more prevalent in host-seeking nymphs than in engorged ones collected from white-footed mice (Peromyscus leucopus) and meadow voles (Microtus pennsylvanicus) over the same time period (χ² = 39.47, P < 0.001). We suggest that I. hookeri is likely to overwinter as non-embryonated eggs inside nymphal I. scapularis and the animal host of the tick may play an important role in attracting female wasps to attached ticks for oviposition. Furthermore, wasp parasitism may have inhibitory effects on the host attachment of the tick.