The acid protease B (SLB) of Scytalidium lignicolum was reduced and carboxymethylated and then subjected to tryptic digestion. Five fragments were isolated and some of them were further digested with alpha-chymotrypsin, thermolysin, and dilute acetic acid. The sequence analysis of these fragments and the peptides by conventional methods established the complete amino acid sequence of SLB. The enzyme was composed of 204 amino acid residues with threonine and valine as its amino- and carboxyl-termini, respectively. Locations of three disulfide bridges were also established to be Cys47-126, Cys140-163, and Cys192-201 by enzymatic fragmentation of the denatured and unmodified SLB. Only a slight homology was found in the sequences of SLB and other acid proteases hitherto reported.