Abstract.1The larvae of the multivoltine imported willow leaf beetle,Plagiodera versicolora(Laicharting), are found in larval aggregations during the first half of the larval stage. An experimental study was conducted to test for seasonal (brood) and group size effects on larval fitness. For each of the three broods, egg group sizes of1, 4, 8, 16, 24 and 32 were established in the field and monitored for the first 6 days post‐hatching.2There was no effect of group size on larval weight, but there was a significant effect of group size on survival. Individuals from group size 16 had the highest survival, which coincides with the mean natural cluster size of 15.3 eggs.3There were no significant interactions between brood and group size.4The effect of larval weight on adult fitness was determined in the laboratory and the field. Larval weight was correlated with egg production in the laboratory but not in the fiel