Eight strains of the genus Aureobasidium obtained from culture collections were tested for their capability to produce poly(beta-L-malic acid) (PMA). Four of the tested strains showed positive results. The most productive strain, A. pullulans CBS 591.75, was used to study the production of PMA in stirred-tank reactors. It was found that PMA was mainly produced in the late exponential phase, and the production related positively to glucose consumption. At the beginning of the fermentation the pH increased from 4.0 to about 7.0; subsequently the pH decreased and remained stable at around 3.0-3.5 for several days. Temperatures higher than 25 degrees C were detrimental to PMA production and cell growth. PMA production and cell growth at 20 degrees C and 25 degrees C exhibited no significant dif ferences. PMA production and cell grow rh were studied under pH-controlled fermentation (at pH 2.0, 4.0, 5.5). The highest PMA production occurred at pH 4.0. PMA production was reduced at pH 2.0 although quite reasonable cell growth occurred at this pH value. Under optimized conditions 9.8 g PMA/l was produced during 9 days of fermentation in the stirred-tank reactors with an overall yield of 0.11 g PMA/g glucose. A procedure for the isolation of PMA and its separation from the other components of the fermentation broth was developed. The isolated PMA was characterized by H-1 and C-13-NMR spectroscopy as well as by infrared absorption spectroscopy. Gel-permeation chromatography revealed a relative molecular mass of approximately 3000-5000 by comparison with polyethylene glycol standards.