1.Genetic polymorphism in the gene phosphoglucose isomerase (Pgi) encoding for a glycolytic enzyme has been shown to affect many traits in adult insects, including flight metabolism, running speed, fecundity and longevity, but it is not known to what extent Pgi genotypic effects are consistent across different life stages. 2.In the Glanville fritillary butterfly, heterozygous AC adult individuals for a single nucleotide polymorphism (SNP) (AA111) in the coding region of Pgi have superior fitness to the common homozygotes (AA) in practically all life-history traits. 3.Here, we studied associations between Pgi SNP AA111 and larval and pupal weights, larval development time in three different temperatures and adult longevity. Small body size and limited mobility of larvae offer little buffer against changes in ambient temperature; hence, temperature is expected to affect greatly larval growth and development. 4.In contrast to adults, larval performance was superior in AA homozygotes in two respects. First, survival was higher in AA homozygotes under stressful conditions, represented by the low-temperature treatment in which survival was generally low. Second, the AA homozygotes had heavier pupae. In spite of the latter result, adult life span was longer in the AC heterozygotes, in support of previous studies. 5.The results on larval growth are consistent with the hypothesis of a trade-off between thermal stability and kinetic efficiency between the different isoforms of the PGI enzyme, but the results also indicate unexpected differences in the genotypic effects at different life stages.