Abstract
1. It has been shown that there is a difference between the resistance of male and female mice to infection withNematospiroides dubius.2. More parasites were harboured, during both the larval and adult parasitic phases, by male mice.3. These worms were found to occupy a similar relative length of the intestine between the stomach and the caecum in male and female mice infected for either 5 or 10 days.4. The relative length of the intestine infected on the fifth day was significantly greater than that infected on the tenth day.This investigation was carried out during the tenure of a Research Studentship from the Department of Scientific and Industrial Research. I should like to thank Professor I. Chester Jones, in whose department the work was done, for the facilities provided and Dr E. T. B. Francis for his helpful and critical supervision.