The effect of water activity (a w) temperature and incubation time on aflatoxin B 1 production by Aspergillus parasiticus NRRL 2999 in soybeans was determined in experimental conditions resembling post-harvest environments. Very low quantities of aflatoxin B 1 were detected at 15°C and 0·902 and 0·950. At this temperature and a w 0·990, competitive Penicillium growth was observed. The minimal a w for aflatoxin B 1 production in this substrate was near 0·865. At this a w level, very low quantities were observed at temperatures above 20°C. At lower a w values (0·801 and 0·750) no aflatoxin B 1 was detected at 15, 20, 25, 30 or 37°C. The optimal a w for aflatoxin accumulation was 0·990, except at 37°C, in which case the optimum value was 0·950. There was a combined temperature and a w effect, since the limiting a w, for aflatoxin production depends on the temperature. The aflatoxin B 1 accumulation at different incubation times was greatly influenced by a w and temperature. At 20°C and an a w of 0·950, aflatoxin B 1 increased with incubation time, at 30°C and an a w of 0·990, toxin concentration decreased as the incubation time increased and at 37°C and an a w of 0·950, it reached a maximum and then decreased.