I . T RY L AN D, M . P OM M EP UY A ND L. F IK SD A L. 1998. The effect of chlorine on b-Dgalactosidase activity of sewage bacteria and Escherichia coli was studied. b-D-galactosidase activity of sewage was more resistant to chlorine than faecal coliform cultivability. At low initial dosage (0·05 mg Cl 2 l −1 ) neither cultivability (colony-forming units (cfu)), nor enzyme activity of E. coli suspensions were severely impaired. When initial chlorine concentration was increased to 0·1 mg Cl 2 l −1 , the cfu number decreased whereas enzyme activity remained high, i.e. the enzyme activity calculated cfu −1 increased. At higher chlorine doses both cfu and enzyme activity were reduced, but noncultivable cells retained assayable activity after chlorination. Mean values of the enzyme activity calculated cfu −1 decreased when the chlorine dosage was increased from 0·1 to 0·5 mg Cl 2 l −1 , but were not significantly different (P × 0·05) for dosages of 0·2-0·7 mg Cl 2 l −1 . After chlorination, b-D-galactosidase activity of E. coli was less reduced than cfu and direct viable count numbers, but more reduced than 5-cyano-2-3, ditolyl tetrazolium chloride and total cell counts, and the enzyme activity represented an alternative activity parameter of chlorinated samples.