The thermophilic fungus Thermomyces lanuginosus was cultivated in shake flasks for up to 120 h with low molecular weight dextran as carbon source supplemented with either Tween 80 or Triton X-100. Addition of Tween 80 to the growth medium gave a 2.7-fold increase in maximum alpha-amylase activity as compared with controls. Triton X-100 did not affect alpha-amylase production. In the presence of Tween 80, the alpha-amylase secretion was stimulated from the third day of cultivation whereas the general protein secretion was increased after only 24 h of cultivation. The amount of biomass produced increased slightly by high concentrations of Tween 80 but decreased by the addition of Triton X-100. Maximum total extracellular protein increased more than three-fold with increasing concentrations of Tween 80 in the medium. Neither the hyphal growth unit length (G) nor the hyphal diameter were affected by Tween 80. Tween 80 did not change the degree of glycosylation pf the alpha-amylase. The addition of Tween 80 appears to give a general stimulation of protein secretion.