The ultrafiltration process is successfully used in wastewater reuse systems. It will also be used widely in drinking water supplies. In wastewater reuse and for safer drinking water supplies the potential for virus rejection by membrane processes is advantageous. Rejection of viruses was examined experimentally by using coliphage Qβ as a tracer. Several types of ultrafiltration membranes with molecular weight cutoff size of 20,000-40,000 were tested. All of these membranes leaked phages to some extent, contrary to the expectation from nominal cutoff size. Penetration of virus through ultrafiltration membranes was theoretically examined by taking pore size distribution of the membranes into account. Modified pore theory and a log-normal pore size distribution model gave an unsatisfactory explanation of the results. The presence of abnormally large pores which are not included in the main distribution was indicated.