Using mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine, a number of homoserine auxotrophs have been isolated from a glutamate-producing Arthrobacter globiformis excreting L-lysine in good amounts. For further improvement, mutants resistant to the lysine analog S-(2-aminoethyl)-L-cysteine have been isolated from homoserine auxotrophs. For the three potent mutants tested, White's medium was found to be the best. Glucose, ammonium nitrate and biotin were found to be optimum at 280 mmol/L, 40 mmol/L and 22 nmol/L, respectively. With optimal glucose, ammonium nitrate and biotin, the strain AECrVI yielded 36 g lysine per L in flask culture.