AbstractFor each of the three species of leeches and four species of triclads inhabiting the stony littoral of 100 British lakes of diverse trophic status, numbers, biomass and mean weight were correlated against a wide range of physical, chemical and other variables. Very few significant correlations were found between the various biological variables and the non‐chemical variables. Significant, positive correlations were obtained for all leech and triclad variables against all of the chemicals, with the exception ofErpobdella octoculatanumbers,Helobdella stagnalismean weight and all of thePolycelis nigravariables which did not correlate with any. Relationships between biological variables and calcium content are given special consideration. Over the range of lakes investigated, total triclad numbers exceeded leech numbers but the reverse was usually true for total biomass. Correlations between the various leech and triclad variables are examine