ABSTRACTPopulations ofFlammulina velutipesadapted to drier environments produce basidiocarps that emerge from their substrate with pilei and stipes 24% broader and pilei 19% thicker than the basidiocarps of populations of the same species adapted to wetter environments. The dry-tolerant isolates had more surface area exposed to evaporative loss, a higher percentage of water in their tissues, and a larger initial water allocation. Both wet-specialist and dry-tolerant population isolates are genetically programmed for large ultimate size under saturated water conditions; however, under water stress conditions, expanding and thickening the pileus too late reduces spore production fivefold for the wet-specialist populations.