in northern communities where source water may be frozen for parts of the year or wastewater is treated using freezing technology, the efficiency of uv inactivation may be affected. this research was carried out to investigate the response of escherichia coli, enterococcus faecalis and spores of bacillus subtilis to uv irradiation after freezing and possible photoreactivation after the bacteria were exposed to uv. the inactivation of e. coli (doses 1.5 - 9mj/cm2), e. faecalis (6 - 25mj/cm2) and b. subtilis spores (20 - mj/cm2) by ultraviolet light was investigated using a collimated beam uv apparatus.