The specific activity and enzyme protein concentration of the developmentally regulated enzyme glucose 6-phosphate dehydrogenase (G6PD) were measured in the developing aggregates and supporting mycelium of a fruiting-impaired variant strain of Agaricus bisporus. The nonregulated enzymes mannitol dehydrogenase (MD) and hexokinase (HK) were assayed for comparison, G6PD activity was higher in aggregates than in the mycelium, whereas MD and HK activities varied little between mycelium and aggregates. Enzyme protein levels varied in a way different from enzyme activity, suggesting the presence of inactive enzyme at times during development. The raised level of G6PD in aggregates provides a possible mechanism for the increased mannitol concentration previously observed in aggregates. There was no parallel to the rapid increase in G6PD activity associated with primordium development of normally fruiting strains growing on compost.