Abstract Macroinvertebrate populations including Gammarus pulex and Asellus aquaticus were surveyed at riffle sites on four lowland rivers, i.e. the Rivers Adur and Ouse and the Chess Stream, Sussex, and the Eridge Stream, Kent. Sites were situated both upstream and downstream of sewage treatment works. Macroinvertebrates were collected using a Surber sampler and at some sites the water analysed for the main chemical constituents. Poor water quality is characteristically associated with low biotic scores and in this respect the Gammarus : Asellus ratio applied to riffle data corresponded with the Chandler Biotic Score Index and the Extended Trent Biotic Index. The Gammarus : Asellus ratio showed the closest correlations with concentrations of BOD, ammonia-nitrogen (NH 3-N), nitrite-nitrogen (NO 2-N) and phosphate-phosphorus (PO 4-P) being sensitive to changes in water quality brought about by organic enrichment in the four rivers investigated. It is proposed that the Gammarus : Asellus ratio may provide a simple biological tool which could be used by anglers and other non-professional river users to routinely monitor water quality.