Abstract 1. Glanville fritillary butterfly (Melitaea cinxia) females lay up to 10 clutches of 50–300 eggs in their lifetime. Clutch size is an important life-history trait as larval group size affects survival throughout larval development.
2. Two experiments were carried out in a large population cage in the field to investigate the life-history and environmental correlates of clutch size.
3. Clutch size decreased with the cumulative number of eggs laid previously, increased with both female body weight and the number of days between consecutive clutches.
4. Genotypic differences among females in the glycolytic enzyme phosphoglucose isomerase had a significant influence on clutch size, partly because females of particular genotypes were able to initiate oviposition earlier in the day and thereby take advantage of the most favourable environmental conditions for oviposition.
5. Factors influencing clutch size were partly different in two summers, indicating the modulating effect of prevailing environmental conditions on reproductive performance.