I investigated the effects of food, crowding, and weather on the behaviour of the slugs, Arion ater and Ariolimax columbianus. Field experiments demonstrated that Arion based its migratory response on the density of conspecifics and the quality and quantity of available food. Migration was most rapid from areas of high slug density and scarce poor quality food. The migration response of Ariolimax was exclusively based on density. Although high slug density induced migration by this species, its rate of migration was lower than that of Arion. Migrating Arion ater were heavier than non-migrating slugs. There was no such relationship between body weight and locomotory or migratory ability of Ariolimax columbianus. Both species exhibited seasonal variation in their behaviour. Hot, dry weather during mid-summer curtailed their foraging and migratory activity. Cooler, wetter weather in late spring and late summer lengthened nocturnal activity periods. Average daily temperature and precipitation were poor predictors of migratory activity. Multiple regression analysis of hourly activity patterns and weather showed that, time of day, air temperature, and atmospheric moisture accounted for most of the variation in hourly nocturnal behaviour. Variation in slug density and food acceptability had no effect on population activity in Arion ater. However, the treatments did influence particular kinds of behaviour. Arion provided with poor food moved and rested more, but fed less, than slugs receiving good food. The effects of density were not significant. Poor food and high slug density increased the activity of Ariolimax columbianus. This effect was also evident in the component behaviours of total activity. Ariolimax moved, rested, and fed more in high density situations. These slugs fed more when good food was available, but the other behaviours were unaffected by this factor. On an hourly basis Arion ater's activity pattern appeared to be based on food acquisition. Over days and weeks, however, this species used both population density and food acceptability to decide to migrate. Ariolimax columbianus' hourly behaviour pattern was most closely related to the density of conspecifies. This response to density persisted over days and weeks, and was the most important factor affecting the migratory behaviour of these slugs.