The River Mersey and its Estuary is assessed for the effects of pollution by using biological indicators; a sampling program for certain bacteria indicates that there is often gross pollution, although there is a wide range of observed values on both short time and space scales. This study considers bacteriological contamination in order to supplement existing survey data on heavy metals, dissolved oxygen, biochemical oxygen demand and suspended solids. Water, Air, and Soil Pollution 22 (1984) 241-255. 0049-6979/84/0223-0241502.25.