Fecundity of tussock moth populations varied among plots after the 1st yr (1972) of conspicuous tree defoliation and declined sharply after a 2nd yr (1973) of defoliation. Degree of defoliation in the 1st yr had no significant effect on egg production, although fewest eggs per mass were produced in the most severely defoliated areas. Severe tree defoliation in the 1st yr significantly affected quality of the filial generation. Field populations that had not already collapsed naturally had a 30% drop in fecundity after the 2nd yr of defoliation. The conclusion is that fecundity was influenced by many factors, food-related and otherwise, in the outbreak. Tree defoliation had its greatest effect at exceptionally high population densities when the favored current needles were destroyed early in the summer, placing surviving larvae under a food stress for much of their development.