ABSTRACTThe average number of basidiospores produced upon the basidium of the nominally tetrasporicAgaricus subrufescenscan vary considerably. In the laboratory a decline in average spore number can be induced by refrigerating the sporocarps for several hours. Spore number also fluctuates in naturally occurring material, in a manner which correlates with diurnal temperature fluctuations. Observations on several other species ofAgaricussuggest that this phenomenon is widespread in the genus. These findings raise questions about the life cycle of these organisms, and suggest an explanation for the origin of bisporic, secondarily homothallic species such asA. bisporus.