1. Larvae of Chlosyne janais (Lepidoptera: Nymphalidae) feed gregariously as early instars on the shrub Odontonema callistachyum (Acanthaceae). During the fourth instar, aggregations break up and larvae feed as solitary individuals.

2. The hypothesis that aggregation increases growth rate was tested by raising larvae on intact plants in the field in different group sizes and measuring their daily growth.

3. There was a striking effect of group size on larval growth whereby larvae more than doubled their weight gain by feeding in large rather than small aggregations on intact plants in the field.

4. This group-feeding advantage was lost altogether if larvae were raised on excised leaves in the laboratory, suggesting that large aggregations may facilitate growth either by inducing a nutrient sink or by overwhelming an induced allelochemical response in the plant.

5. Although larval survival was higher in cages that excluded enemies than in exposed aggregations, there was no influence of group size (experimentally manipulated) on short-term survival in the field. However, there was a weak positive relationship between short-term survival and the size of naturally occurring larval aggregations in the field. These data provide mixed support for the notion that gregarious feeding promotes defence against natural enemies.

6. Although the group defence hypothesis warrants further investigation, feeding facilitation is clearly an important factor contributing to the aggregation behaviour of C. janais larvae.