Virus removal in membrane separation processes was investigated by employing coliphage Qβ as a tracer. Several types of microfiltration membrane and ultrafiltration membrane were tested. Two types of filtration experiments were carried out; dead-end filtration and cross-flow filtration. The membrane surface deposits played an important role in the rejection of viruses in the filtration of activated sludge and pond-water, whereas acrylate polymer cake did not affect the rejection of Qβ. The leakage of ultrafiltration membranes was well examined by the high concentration of Qβ applied. The major part of the rejected coliphages were adsorbed onto the membrane and in its surface deposit.