Microorganisms able to bioconvert DL-2-amino-Δ(2)-thiazoline-4-carboxylic acid (DL-ATC) into L-cysteine were originally isolated from 10 soil samples with DL-ATC as the sole nitrogen source. Ninety-seven L-cysteine-producing bacterial strains were screened out and obtained in pure culture. Among them, a strain, designated as HUT-78, was selected as the best producer, with a molar bioconversion rate of 60%. Based on the 16S rRNA gene sequence analysis, this isolate was placed within the genus Pseudomonas. A novel mutant of this strain with a significantly reduced activity of L-cysteine desulfhydrase, a L-cysteine-decomposing enzyme, was derived by UV-mutagenesis. This mutant, designated as mHUT-78, exhibited a 42% increase in L-cysteine producing activity. Moreover, the bioconversion reactions in both the parent and the mutant strain were significantly accelerated by co-overexpression of the two key enzymes, AtcB and AtcC, involved in the bioconversion reaction.