A heuristic approach is employed to demonstrate that rotifer community structure, i.e. similarities in quantitative abundance at various stations as determined by cluster analysis, delineates water masses of different trophic conditions in Lake Huron. Since cluster analysis utilizes the abundance of each and every species at a station, the uniqueness of the species as a variable is retained. Therefore the coefficient of similarity, or Euclidean distance, was interpreted as a relative measure of rotifer community structure. Since cluster analysis of physicochemical data yielded similar station groups which are highly correlated to trophic states, it follows that rotifer community structure in this sense could be an indirect measure of trophic conditions in these waters. Finally, qualitative data on the presence or absence of certain species may have some value at extremes of trophic conditions; however, quantitative data, when utilized by multivariate methods, such as cluster analysis, offer more indicator potential in the wide range of ill-defined mesotrophic conditions.