Pleurotus ostreatus and P. pulmonarius, wood-destroying and edible basidiomycetes, were the subject of an ecophysiological study with regard to the connection between water household in the fruitbody and sporulation. Field observations were combined with controlled laboratory experiments on cultivated fruit-bodies. In the field it appeared that when it is about freezing point during the day, sporulation is mainly determined by temperature factors. In milder periods, water content of the fruit-bodies is the major factor influencing the sporulation. There was not found a daily pattern in sporulation nor endogenic rhythm. In cultivated fruit-bodies, the field observations were confirmed as to the influence of relative humidity for the duration of the sporulation, but a change in humidity during the day did not influence sporulation in a uniform way. In the range of 8—22°C, temperature proved to be the most important factor, always positively correlated with sporulation intensity. Pleurotus is considered as a poikilohydric organism, as its water household is mainly determined by physical laws, not hampered by special structures preventing dehydration, etc. Fruit-bodies proved to be able to tolerate rather long periods of frost or dehydration, starting sporulation again under favourable conditions. The hymenial elements are able to withstand frost and/or drought, which is also demonstrated with SEM pictures of these structures.