SummaryMucosal mast‐cell hyperplasia is frequently observed in intestinal nematode infections and it has been suggested that mast‐cell responses to parasite antigens are involved in worm expulsion (self cure). To evaluate the importance of this mechanism, the course of infection and expulsion of Nippostrongylus brasiliensis was compared in mast‐cell deficient W/Wvand normal (+/+) mice. Initial infectivity rates were similar, but the subsequent kinetics of expulsion of adult worms differed principally in that the onset of expulsion in mast‐cell deficient mice appeared to occur 24–36 h later than that in normal mice. Expulsion was complete by the 14th day post infection in both W/Wvand normal mice. Worm fertility (as estimated by faecal egg output) also differed in W/Wvand normal mice, with maximal egg output in W/Wvmice occurring 24 h later than that in normal mice. Although a few mast cells were present in the intestinal mucosa and tongue of W/Wvmice, their numbers did not change during the course of infection with N. brasiliensis. In contrast, worm expulsion in normal mice was associated with a moderate increase in numbers of intestinal mast cells, commencing at the onset of expulsion and peaking several days after expulsion was