Abstract.1Among‐population differences in pupal mass were studied in a geometrid,Epirrita autumnata.SomeEpirrita autumnatapopulations regularly reach outbreak densities while others are never known to do so. Because adults do not feed, pupal mass of females correlates strongly with fecundity.2Larvae were collected from twelve field sites. Ten of our sample populations originated within the outbreak range of the species and represented different phases of outbreaks. Two populations originated outside the outbreak range.3Pupal mass of field‐collectedE. autumnatavaried significantly among populations. The peak phase populations had the smallest pupae and the biggest were found in low density populations outside the outbreak range.4Offspring of moths from each population were reared under identical conditions in two larval densities. Significant differences were not found in pupal mass among populations. That is, the inherent size, correlated with fecundity of moths, was not different between populations originating within and outside the outbreak range, nor among collections from different densities or phases of the outbreaks.5Rearing density did not interact in a consistent way with population.6As far as size and fecundity are concerned, the results do not support Chitty's hypothesis that differences in genetic composition of the population at low and high density phases generate cyclic fluctuations of population density.7Because no hereditary or maternal differences were found in size and fecundity betweenE.autumnataoriginating within and outside the outbreak range, variation in reproductive capacity cannot explain why outbreaks occur only in some populati