A wild type of Aspergillus sp. ATHUM-3482 produced extracellular polygalacturonase when grown in liquid medium containing citrus pectin as sole carbon source. A number of factors affecting enzyme activity were investigated. Polygalacturonase activities as high as 4.3 U ml(-1) (reducing-group-releasing activity) and 17 U ml(-1) (viscosity-diminishing activity) were obtained under optimum growth conditions. With sugar-beet as sole carbon source the respective activities were 6.5 U ml(-1) and 40 U ml(-1), the highest achieved in this work. Under these conditions no pectin lyase or pectinesterase activity was detected. The above yields of polygalacturonase activity compare favourably with those reported for fungi grown under similar growth conditions.