The changes in numbers of 6 cell populations in the intestine of mice at various intervals after primary and challenge infections with Strongyloides ratti have been quantified. The number of lamina propria mast cells increased 8 days after primary infection and reached a peak at 12 days. After secondary infection, there was a transient fall in mast cell numbers followed by a slow increase. Globule leucocytes showed a similar trend early in the primary infection and had reached normal levels after 28 days. After challenge infection, there was an early and rapid increase in their numbers. Granular intraepithelial lymphocytes did not alter significantly during the first 14 days, but were significantly greater 28 days after primary infection; they did not vary significantly after challenge infection. However, numbers of non-granular intraepithelial lymphocytes increased 10 days after infection, were elevated prior to the secondary infection at 28 days, then declined in numbers nearly 2 weeks after challenge infection. Goblet cells increased significantly 12 days after primary infection then declined rapidly. After challenge infection, there was an accelerated increase in numbers. Eosinophil numbers increased 4 days after infection, reached a peak at 12 days and then declined. After challenge infection, there was an augmented and accelerated increase in eosinophil numbers followed by rapid decline. The role of the various cells types in host defences against worms or in containment of the inflammatory responses evoked by these parasites are discussed.