Niche breadths in animal and plant populations are often measured without regard to the relative frequencies of the various resources available to the organisms. Recent papers have provided more accurate indices for use when available resources can be quantified. These indices may lack wide applicability or simple biological interpretations. We suggest that niche breadth be defined as the degree of similarity between the frequency distribution of resources used by members of a population and the frequency distribution of resources available to them. Similarity can be quantified with the familiar Proportional Similarity (PS) Index. This index, which measures objectively the similarity between two frequency distributions, reflects in satisfactory fashion the breadth of a population's niche.