SUMMARY: Of 4 methods of assessing the bacteriological quality of mussels the semiquantitative method of the Fishmongers’Company was the most useful from the public health aspect where facilities may be limited. A modification of Clegg & Sherwood's method, using pour plates, was more sensitive and more simple to perform than the original method and is to be recommended for routine use. Two Escherichia coli cells/ml of mussel tissue probably represents the upper acceptable limit for satisfactory grading of mussels. The presence of small numbers of salmonellae in purified mussels is probably not highly significant but tests for their presence will yield useful epidemiological data.