Males and females of Nippostrongylus brasiliensis were neither significantly attracted nor repulsed by chemical secretions released from members of the same sex. Although solitary, responding males were strongly attracted to the source of a sex pheromone liberated from females, this response decreased markedly when more than one responding male was exposed simultaneously to the female pheromone gradient or when two or more males were placed together with the pheromone-releasing females. Male response to the female pheromone source decreased steadily as the ratio of males to females at the source was increased from 0:35 to 10:35. However, a further increase in the male: female ratio at the source reversed the inhibitory trend and caused responding males to be attracted again. The unknown interactions between males and females which cause this unexpected phenomenon appear to be accomplished primarily through chemical communication. The role of pheromones in the reproductive biology of nematodes has received relatively little study, even though such chemicals are implicated as a primary cause of premating attraction between the sexes in over 20 nematode species (Anya, 1976). To date, many of these reports are descriptive, apparently representing solitary investigations on various nematode species. However, several groups of investigators have attempted to unravel some of the intricacies of sex pheromone communication for plantparasitic and free-living nematode species. Green, Greet, and co-workers have described several facets of male-to-female attraction in Heterodera spp., including the site of pheromone production, the effect of various pheromone dosages, genetic relationships, and pheromone lability (Green and Greet, 1972; Green and Plumb, 1970; Green et al., 1970; Greet et al., 1968). Working with a Panagrellus species complex, Samoiloff and his associates have contributed information concerning gonodal association with pheromone production, strain variations, locomotor activation, relation of pheromones to population genetics, and preliminary molecular weights of pheromone components (Balakanich and Samoiloff, 1974; Cheng and Samoiloff, 1972; Samoiloff et al., 1973; Samoiloff et al., 1974). Intensive pheromone investigations of zooReceived for publication 12 December 1976. parasitic nematodes are lacking. However, inhibition of male movement in response to female-produced pheromone, mediated by unknown chemical and/or thigmokinetic stimuli, has been reported for two zooparasitic species. Alphey (1971) suggested that thigmokinetic responses among responding males interfered with their attraction to females of Nippostrongylus brasiliensis. Salm and Fried (1973) found that groups of three or five males of Camallanus sp. required more than double the test period to respond comparably to single worms. Salm and Fried also postulated that thigmokinetic reactions among the responding males were responsible for their inhibition of movement, although they did not negate the possibility of short range chemical effects. We considered these inhibitory effects worthy of further study in our attempts to elucidate additional parameters involved in premating sexual communication in Nippostrongylus brasiliensis. MATERIALS AND METHODS Nippostrongylus brasiliensis was maintained by inoculation of white mice (Hilltop Laboratory Animals, Inc.) as previously reported (Bone et al., 1977a). Five-day-old adult worms were used for experimentation, since the production of and response to pheromone were maximal at this age (Bone et al., 1977b). The pheromone bioassay procedure and apparatus were described earlier (Bone et al., 1977a). Briefly, nematodes used as pheromonesource individuals were confined at one end of a