The behaviour of sex-pilli specific (F+) bacteriophage during the depuration process was investigated using Pacific oysters (Crassostrea gigas) and edible mussels (Mytilus edulis) naturally contaminated with untreated sewage. F+ bacteriophage was eliminated considerably slower than E. coli. This effect was most pronounced in oysters which showed average T90 bacteriophage values of 62.5 hours compared with 11 hours for E. coli. F+ bacteriophage in mussels was largely confined to the digestive tract. The majority of E. coli were similarly located but, in contrast, counts were also distributed throughout the other internal tissues. Investigation of ultra-violet (UV) dosage showed that the difference in depuration rate was unlikely to be due to the higher resistance of F+ bacteriophage to UV irradiation. These results demonstrate the potential usefulness of F+ bacteriophage as an alternative indicator organism for depuration of shellfish.