Secondary effluent was irradiated with UV in a shallow bed reactor at flow rates between 10 and 30 m3. h-1using 1–3 high pressure mercury lamps. UV doses were calculated to range between 2.5 and 25 mJ.cm−2and inactivation of four groups of microorganisms was measured: E. noli, fecal streptococci, somatic coliphagesi and F–specific bacteriophages. Their relative UV resistance (in this order) was 1.0 : 1.5 : 1.1 : 2.3. Inactivation of F–specific (coliphages with 3 log10units required a dose of approx. 36 mJ.cm−2. Dose–response curves were linear with correlation coefficients of ca. 0.90, except for somatic coliphages, which seem to include a relatively UV–sensitive and a more resistant fraction. This study clearly demonstrated that F–specific bacteriophages are better indicator organisms with regard to viral inactivation by UV than classical parameters like E. coli. fecal streptococcior coliphages.