Water Company Europoort started a pilot plant (MP)ultraviolet (UV) study to determine the UV- fluence to meet the Dutch drinking water standards. The results of large volume sampling demonstrated that environmental spores of sulphite-reducing clostridia (SSRC), were highly resistant against UV. With the pilot plant at a flow of 180 m3/h a challenge test was conducted to compare the susceptibility of environmental SSRC and lab-cultured spores of C. perfringens, MS2 bacteriophages and Bacillus subtilis. The latter was dosed as a biodosimeter with a calibrated UV253.7nm sensitivity to confirm the biocidal fluence. This test demonstrated that environmental SSRC were a factor of 1.6 more resistant against UV than the lab-cultured spores of C. perfringens. Furthermore, comparing the inactivation of these environmental SSRC indicated that they are more resistant to UV than other relevant micro-organisms for drinking water safety. Combining large volume sampling for SSRC and biodosimetry yielded an instrument for on-site verification of the efficacy of the UV-system. Further research is necessary to determine if this relation is consistent in this and other waters and the cause of the increased resistance of environmental micro- organisms. Includes 12 references, tables, figures.