Production costs for shiitake (Lentinula edodes) are on the rise in the United States due to increasing expenses including materials, labor and energy. Increased yield and improved bioconversion of raw materials may improve grower profit margins and may help reduce the cost of shiitake to the consumer. Two crops (Crop 1 and 2) of shiitake were grown to evaluate effects of three substrate moisture contents (50%, 55% and 60%), two log weights (2.7 and 3.2 kg) and three porosities of bag filter (low, medium and high) on mushroom yield (g/log) and biological efficiency (BE). Yield data were collected under controlled environmental conditions for two breaks. The formulation with 55% substrate moisture gave the highest yield and BE. Higher mushroom yields were produced from heavier logs (3.2 kg), but BE was not significantly affected. Filter porosity significantly affected yield and BE in Crop 1 but not in Crop 2. Significant interactions were observed for log moisture content x filter porosity for both crops. There were no significant two-way interactions observed for filter porosity x log weight or three-way interactions observed for moisture content x filter porosity x log weight. Maximum yields were obtained from 3.2 kg logs with a substrate moisture content of ca. 55% using medium or low porosity-filtered bags. This study may provide growers with additional information to better optimize production practices and become more efficient and competitive. (C) 2008 Elsevier Ltd. All rights reserved.