SummaryWhen adult worker honeybees in small hives, kept in screen flight cages, were fed sucrose syrup containing 5%, 10%, 15% of lactose or 2·5%, 5%, 7·5% of galactose, galactose was found in their faecal droppings. The quantity of faecal galactose increased as the feeding period increased. Significantly more faecal galactose was found in bees fed 10% and 15% of lactose than in bees fed 5% (P<0·05 in both cases), further indicating the stimulating effect of lactose feeding on activity of lactase (β-galactosidase). However, significantly higher mortality (P<0·05) occurred in bees fed 10% and 15% (but not 5%) of lactose than in control bees. The data suggest that the bees could tolerate no more than 5% of lactose.Bees fed galactose at various concentrations had significantly higher mortality than the control bees (P<0·001 in all cases), although large amounts of the ingested galactose were excreted during cleansing flights. Presumably galactose interferes with carbohydrate metabolism of bees, and is thus probably toxic to bees.