Mycelial growth of Agaricus bisporus on sterilized compost is strongly stimulated by preincubating the compost with the thermophilic fungus Scytalidium thermophilum. This stimulatory effect is not species specific, for either A . bisporus or S. thermophilum. Normal mushroom compost is almost completely colonized with S. thermophilum. In experimental composts a positive relation was found between the logarithm of mushroom yield of A . bisporus and the density of S. thermophilum. S. thermophilum provides for compost selectivity: it protects against negative effects of compost bacteria on mycelial growth of A . bisporus. S. thermophilum is inactivated by the growth of A . bisporus mycelium. INTRODUCTIONThe edible white button mushroom Agaricus bisporus (Lange) Imbach is cultivated in the western hemisphere on a substrate consisting of a composted mixture of horse manure, wheat straw, chicken manure and gypsum. Conventionally two composting phases are distinguished (Sinden & Hauser, 1950;Fermor et al., 1985). Phase I is an outdoor fermentation process during which NH3 and unpleasant-smelling compounds are emitted into the environment. Phase I1 is an indoor process of pasteurization followed by conditioning at approximately 45 "C. This can either be performed in limited quantities in mushroom houses or in bulk in 'tunnels'. In mushroom houses self-heating of the compost is controlled by ventilating air around layers of up to 30cm thick contained in trays or beds. In tunnels air is forced through the body of the compost, allowing a more accurate control, and layers up to 250 cm can be processed (Derks, 1973 ;Gerrits, 1988 a).If phase I were carried out indoors waste air could be treated before discharge, preventing the release of ammonia. A combined phase I and phase I1 indoor process as introduced by Lambert (1941) could prevent further environmental problems as preliminary results of our group P. J. L. Derikx, personal communication) indicate that the release of fetid volatiles is prevented in phase I1 carried out in tunnels. Recent experiments suggest that combined phase I and phase I1 indoor composting is feasible (Laborde et al., 1987;Perrin & Gaze, 1987;Gerrits, 1987). Perrin & Gaze (1987 and Gerrits (1987) processed raw moistened materials in tunnels and followed phase I1 procedures only. Good mushroom yields were obtained.The basic nutritional requirements of A. bisporus in vitro are already known (Wood & Fermor, 1985) and easily met by compost. However, successful colonization and use of compost by A. bisporus depends strongly on ecological properties, usually referred to as 'compost selectivity'. It is largely unknown how selectivity can be described in chemical or biological terms. Obviously the rate of substrate colonization during the vegetative phase of mushroom culture is important.