Submerged cultures of Penicillium notatum on a modified Czapek-Dox medium, containing sucrose or glucose, were characterized by high intracellular and extracellular glucose oxidase activity, heavy gluconate production and failure to sporulate. Addition of Ca 2+ to cultures within 48 h of inoculation promoted sporulation, greatly reduced the rate of increase of glucose oxidase activity in the biomass and medium and, in turn, the production of extracellular gluconate. Gluconate media supported slow growth of the fungus and a low level of sporulatiom even in the absence of Ca 2+ but the addition of Ca 2+ enhanced spore formation. Fructose media, on which glucose oxidase and gluconate were not produced, supported Ca 2+ -induced sporulation which was, as with sucrose and glucose, entirely dependent upon the presence of Ca 2+ within 48 h of inoculation; thereafter the Ca 2+ effect was not manifested. The possible role of gluconate as a regulator of sporulation is discussed, along with views that ageing of components of the biomass may exert a primary effect upon the sites of the Ca 2+ trigger.