Attraction between adults of Nippostrongylus brasiliensis was studied both in vivo and in vitro particularly with regard to the effects of host immunity on the behavior of the parasite. Most worms were found in clusters in the intestines of larval-infected rats but the number of isolated worms, particularly females, was greater in 14-day (immune) than in 7-day infected (nonimmune) hosts. Intubation of small numbers of normal adults into uninfected rats resulted in recovery of mostly aggregated worms unless infections consisted only of males. In contrast, immune-damaged worms exhibited little aggregation regardless of the sexes of worms instilled. Analysis of pairing between worms in vitro indicated that attraction occurred in the absence of host factors between all permutations of normal worms except male-male combinations. Pairing between damaged worms occurred only between males and females and not between worms of the same sex. Attraction to worm excretory and secretory products (ES) indicated that chemical factors mediated pairing. Normal female ES was attractive to both sexes whereas damaged female ES failed to attract either sex. ES from both normal and damaged males attracted females but not males thereby confirming the results of other experiments.