Enrichment cultivation was used to select a bacterial strain capable of metabolizing glucoisosaccharinic acid (GISA), the major organic acid component of black liquor from the sulphate cellulose (Kraft) process. The bacterium was isolated from mud at the bottom of a lake polluted by pulping effluents. It grew well on GISA as the sole carbon source at pH 6.0±0.5, the absolute limits for growth on this substrate being 5.1<pH<7.2. It was, however, unable to metabolize glucose under any of the conditions tested. Glucoisosaccharinic acid at an initial concentration of 2 g l-1 was consumed within 24 h in batch laboratory fermentations, the maximum observed rate of consumption after the lag phase being almost 10 g l-1 d-1. The strain appeared to be independent of organic nitrogen.