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—Determining the importance of prey taxa in the diets of predacious species is a frequent objective in fisheries research. Various indices of prey importance are in common use, and all give different results because of their emphasis on different aspects of fish diets. We explored these differences by empirically comparing four well-known indices—percent weight (%W), percent occurrence (%O), percent number (%N), and percent index of relative importance (%IRI)— as well as a modified %IRI (%MIRI), as applied to an extensive data set on the diets of six fish species in Spirit Lake, Iowa. Correlations among all indices were positive but were weaker among component indices (%W, %O, and %N) than between the two compound indices (%IRI and %MIRI); correlations among component indices were also weaker than correlations of compound with component indices. Correlation strength of %MIRI with the three component indices varied greatly (%N , %O , %W), whereas the correlation strength of %IRI with component indices was similar. Importance values based on %W, %MIRI, and %N depend more on prey size than those based on %IRI and %O. The %W and %MIRI emphasized the importance of large prey taxa, whereas %N emphasized small prey in diets; %IRI and %O were similarly unbiased with respect to prey size. The %O yielded substantially higher importance values than all other indices. Thus, for use as a general index of dietary importance, we believe %IRI provides the optimal balancing of frequency of occurrence, numerical abundance, and abundance by weight of taxa in fish diets. Importance of prey taxa is a frequently used quantitative description of fish diet and the relationship of diet to other characteristics of fish populations and communities (Bowen 1996). Biologists are interested in the importance of prey taxa to predator growth, consumption, competition, predation, and relationships between prey importance and availability. Accurately quantifying importance of prey taxa and understanding the contributions of various prey to predator well-being is essential for effective management of fisheries resources (Bowen 1996). Several indices have been used to express the relative importance of prey taxa, including percentage by number (%N), weight (%W), volume (%V), and occurrence (%O); index of relative im* Corresponding author: cpierce@iastate.edu Received January 3, 2000; accepted January 14, 2001 portance (IRI); and modified index of relative importance (MIRI). Among them, %W (or %V) has been the most popular index to describe prey importance and its relationships with fish well-being and prey availability (e.g., Hubert and Sandheinrich 1983; Wahl and Stein 1993; Hartman and Brandt 1995; Rand and Stewart 1998; Persson and Hansson 1999), followed by IRI (e.g., Sammons et al. 1994; Cortes et al. 1996; Swenson and McCray 1996). Occasionally, %O, %N, and MIRI are used as indices of prey importance (e.g., Pitcher 1980, 1981; Chapman et al. 1989; Bridcut and Giller 1995; Gaughan and Potter 1997). No index is without bias, and biases attributed to dietary indices have stimulated different opinions on their use for several decades (Hynes 1950; Berg 1979; Wallace 1981; Cortes 1997; Hansson 1998). There are two major opinions in this literature. One is to use a component index such as %W, %N, or %O, chosen largely on the basis of