AbstractClearing of agar plates containing ball‐milled, delignified straw has been used for screening mutants ofPenicillium funiculosumIMI 87160 iii. The effects of glycerol and a number of sugars on the clearing were investigated for selecting derepressed mutants. The β‐glucosidase synthesis by one such mutant, C22c, in shake flasks containing straw was not repressed by 5% glycerol, whereas activities on filter paper, CM‐cellulose, andp‐nitrophenyl‐β‐xylosidase were only partially derepressed; xylanase was extensively derepressed. The evidence for separate control of the enzymes involved in the solubilization of straw