An extracellular endo-polygalacturonase (PGL) from the psychrophilic fungus, Mucor flavus was purified to homogeneity by two ion-exchange chromatography steps using CM-Sepharose CL 6B. The PGL with a molecular mass of 40 kDa by SDS-PAGE, 3 8.7 kDa by mass spectrometry and a pI above 8.3 was optimally active at 45 degreesC and between pH 3.5 and 5.5. It was stable up to 40 degreesC for 4 h and between pH 2.5 and 6.0 for 20 h at 20 degreesC. Activity and stability of PGL decreased rapidly above 50 degreesC. The PGL was active on polygalacturonic acid and esterified pectin, but the activity on pectin decreased with an increase in degree of esterification. The enzyme rapidly decreased the viscosity of polygalacturonic acid and 89% esterified pectin, but the amount of reducing sugars released from polygalacturonic acid was double that of from 89% esterified pectin. The PGL released a series of oligo-galacturonates from polygalacturonic acid and pectin. The concentration of oligo-galacturonates with DP above 5 was initially high and decreased with time. The enzyme preferentially hydrolysed the oligo-galacturonates with DP 5 and above in an endo-fashion and appeared to possess 6-7 sugar binding sites in its active site. A 39 amino acid sequence at the N-terminal of this PGL showed only 17-29% sequence identity with PGLs of glycoside hydrolase family 28.