SummaryThe histopathological responses to the nematodeStrongyloides rattihave been studied in rats following single and multiple percutaneous infections. Under these conditions, rats developed an effective immune response which was shown to be retrophasic, i.e. during primary infection, adult worms in the small intestine were the main target of the response: during secondary infection, the intestinal and, more strongly, the pulmonary phases were affected; following multiple infections, the response was principally effective during the cutaneous phase. An increase in the number of tissue eosinophils in all these sites was a common feature which varied according to the level of immunity acquired by the host. There was an increase in the percentage of degranulated mesenteric mast cells which coincided with the expulsion of the intestinal phase during primary and secondary infections. No direct contact between eosinophils and any stage of the parasite was seen in the small intestine or lungs of normal and immune rats during primary and secondary infections. However, in the skin of hyperimmune rats, eosinophils increased rapidly (within 3–6 h) forming eosinophil‐rich granulo‐mata in which eosinophils were seen in close contact with the cuticle of the invading larvae. This may suggest anin vivoeffector role for these cells. This and other possible roles of eosinophilsin vivohave been discussed in the light of thein vitroaction of eosinophils in other parasitic systems. Both the tissue eosinophil response and the percentage of degranulation of mesenteric mast cells, during the intestinal phase, were affected following treatment with corticoste