1. Epizootics of nucleopolyhedrovirus (NPV) are an obvious component of the population fluctuations of several species of temperate forest Lepidoptera, including the western tent caterpillar, Malacosoma californicum pluviale (Dyer). An observed relationship between epizootics and the subsequent reduction in fecundity of populations led to the formulation of the disease defence hypothesis. This hypothesis predicts that viral epizootics in peak populations select for more resistant moths and that their reduced fecundity in declining populations reflects a cost of disease resistance. 2. To test the disease defence hypothesis, we carried out bioassays to measure the variation in larval resistance to NPV infection for families of western tent caterpillars from four spatially distinct populations over 3 years of peak and declining host densities. 3. Each female moth lays a single egg mass and larvae are gregarious and remain together through development. We found that the resistance to disease of larvae within families was not related to the number of eggs in the mass from which they hatched (the fecundity of their mother). 4. Disease resistance of larvae varied among populations and over time in a manner consistent with selection for resistance. One population that had not experienced a strong viral epizootic during the last population decline was more susceptible to infection in the first year of the study. Larvae from a second population that experienced an early epizootic became significantly more resistant. The resistance of two other populations increased slightly before the viral epizootic occurred in the field however, and thus could not be explained by selection. 5. As population densities declined from peak density, the background mortality of larvae increased and the fecundity of moths decreased. This indicates a general deterioration in the quality of field populations of tent caterpillars associated with the declining populations. 6. Although some evidence suggests that viral epizootics can select for increased resistance of field populations of tent caterpillars, the general deterioration in quality, elevated background mortality, and the reduced fecundity after the epizootic are stronger influences on the population decline. These are possibly related to sublethal viral infection.