Freshly formed wild type Dictyostelium discoideum spores are constitutively dormant, and thus require an activation treatment to germinate. Wild type spores may germinate without an activation treatment (autoactivate) after a period of ageing (maturation) in the intact fruiting body. Mutants have been isolated which autoactivate without the need for ageing. Autoactivation of mutant and aged wild type spores appears to occur by identical mechanisms; thus the mutation may involve premature maturation. Autoactivation is mediated by autoactivator substances released from spores as they spontaneously swell. These factors are readily chromatographed, and elute from a Biogel P2 column in three peaks of activity. One activity peak appears only after spores have begun to germinate. No autoactivator substances are released from heat activated spores. Autoactivation is sensitive to cychloheximide, and may result from altered spore permeability. Autoactivation is likely to be the mechanism of D. discoideum spore germination in nature.