An apiary of 24 overwintered colonies of honeybees (Apis mellifera) was divided in the spring into 2 groups so that each group contained weak, medium and strong colonies with similar amounts of brood. All colonies were fed with sugar syrup and each colony of one group received in addition 70 mg fumagillin. Treatment with fumagillin increased both brood production and length of worker life by 20%, resulting in a 40% increase in colony population. Fumagillin treatment also increased unit honey production by 19%, so that the total increase in honey surplus amounted to 58%. Unit productivity was not correlated with any of the 7 parameters of colony strength that were measured and so should be treated as an independent genetic character.