Newly hatched larvae of the cyprinid Leuciscus leuciscus in the River Frome, England, commenced feeding on rotifers and diatoms but expanded their repertoire to copepods, cladocerans, and chironomids as they grew. Daytime gut contents averaged 3–4 times the volume present during 0200–0600 hours. A detailed study was made of gut-content variation among 3-week-old larvae from four backwater sites; each river site was sampled 10 times in 31 h. Forty-seven percent of variation in food volume was attributable to differences between individual larvae at the same site and time. The remaining variation was due principally to differences among sites (22% of variance) and sampling time (24%). Variation in gut prey assemblages was due mainly to among-site differences although time and site x time interactions also contributed significant amounts of variance. These results indicate the importance of rigorous sampling strategies and analysis in characterising larval diets. Data generated from a single time or particularly from a single site cannot be reliably applied to a wider context.