Abstract. 1. We studied the fecundity of Epirrita autumnata Bkh. (Lepidoptera) in relation to pupal weight of females and males.
2. There was no clear correlation between male size and the success in fertilization over a considerable range of weights (from 50 mg to over 100 mg) although in one experiment only large males were successful in multiple fertilizations.
3. In the three experiments where small males (<50 mg) were used they showed reduced performance, and a large proportion of them was sterile.
4. Unlike in males, the number of eggs laid by females correlated linearly with pupal weight over the whole range of female weights.