Agaricus bisporus and Agaricus blazei are commercially produced on composted organic substrates. To test if the environmentally problematic composting process is necessary, grain substrates were examined for mushroom production. The basal substrates consisted of sterilized millet grain to which various oilseeds (niger, safflower, and soybean) were added at rates of 0%, 15%, and 30% of the total substrate composition. Furthermore, the effect of the commercial delayed-release supplement S41 added to the substrate at 0% and 5% of total composition was tested for its effect on yield. The addition of oilseeds to the basal substrate is termed Stage I supplementation, whereas the addition of S41 before casing is termed Stage II supplementation. Mushroom production was highest (16.9 kg m- 2) for A. bisporus when grown in the basal grain-based substrate supplemented with 15% soybean and 5% S41. For A. blazei, the highest yield (15.9 kg m- 2) was obtained with the addition of only 30% niger to the basal substrate. In almost all treatments of A. bisporus, an increase in yield when using the S41 supplement was observed. However, this effect was not comparable with the A. blazei trials. The magnitude of the yield increase due to the S41 amendment was dependent on the type and rate of oilseed used. Additionally, a complete inhibition in the growth of A. blazei was observed for all treatments involving 30% soybean. The results indicate that A. bisporus and A. blazei can be cultivated on sterilized mixtures of grain and oilseeds as substrates, but the two mushroom species have significantly different nutritional requirements for optimal fructification. Finally, a combination of Stage I and Stage II supplementation can be used to maximize yield from non-composted grain-based substrates.