Piscivorous fish are size-selective predators. Although sizes of prey selectively ingested by piscivores traditionally have been measured in terms of prey length relative to predator length, the relationship between prey body depth (measured dorsoventrally) and piscivore mouth gape may be a more appropriate measure of prey size selection. In 2-d feeding trials with three sizes of largemouth bassMicropterus salmoides, I offered various sizes of shallow-bodied fathead minnowsPimephales promelasand deep-bodied pumpkinseedsLepomis gibbosusin assemblages of one or both species. All sizes of predators preferred pumpkinseeds with body depths well below the maximum size ingestible. Small predators also preferred fathead minnows with body depths below the maximum size ingestible, whereas intermediate and large predators selectively ingested the largest fathead minnows offered. Largemouth bass never ingested prey of body depth greater than their own external mouth width. Although lengths of selectively ingested fathead minnows and pumpkinseeds differed, largemouth bass showed highest preferences for prey of similar body depths regardless of taxonomic identity. These results suggest that, in addition to setting constraints on maximum sizes of prey that can be ingested by piscivores, the relationship between prey body depth and piscivore mouth gape may also be important in selection of prey within the range of ingestible sizes. Therefore, body depth may be more useful than the traditional measure of prey length as a common measure for examining prey selection by gape-limited piscivores over a wide array of prey species.