Because the most commonly used measures of prey quantity employed in fish dietary studies have limitations, each emphasising different aspects of the diet, several authors have preferred the use of compound indices for the purposes of prey ranking. However, the indices described so far are very heterogeneous and produce results that are rather difficult to compare. In this paper, a generalised index is described which is based on vector geometry. and can be used with any number, type and combination of percentual parameters. A method for the graphic presentation and interpretation of the results produced by stomach content analyses is also presented.