Abstract Computer programs were employed to merge the data from a series of samples of macro-invertebrate fauna and to calculate cumulative values of Trent Biotic and Community Diversity Indices. Whilst the values of both indices are related to the degree of pollution present in the water they are also dependent upon sample size. It is suggested that water, quality may be adequately assessed by either index value calculated for a standard sample containing 1000 organisms. When cumulative data are pooled from a range of locations differing widely in the degree of pollution it is apparent that the two indices are highly correlated with each other. It is concluded that the Trent Biotic Index is a semiquantitative rather than a qualitative parameter.