We examined the consistency of diel patterns observed in fish and aquatic invertebrates over three 24-h cycles within a 10-day period in summer (28-29 July, 4-5 and 7-8 August 1997) in the River Lee, Hertfordshire, England. We tested for temporal variations in epibenthos density and population size structure, age-specific fish density, microhabitat use and contagion as well as for correlations between fish length and prey number (in the gut) per millimetre fish standard length, a surrogate of foraging success, and fish density in point samples (an estimator of shoal size). Consistent patterns were observed in invertebrates and fish, though variability was greater in the former. Epibenthic invertebrates (except Chironomidae) demonstrated few or no diel differences in density, but some differences in size (based on head widths) were found in larvae of Chironomidae and Ephemeroptera (mainly Baetis rhodani (Pictet) and Ephemerella ignita Poda) between times of day. Fish density varied consistently and diel variations were observed in fish shoaling behaviour, habitat use, prey size and prey composition. Prey numbers in the gut generally decreased with increasing shoal size, but this was significant in Gobio gobio L. only. In some fish species/size classes, prey numbers correlated either positively or negatively with habitat variables, in particular with the distance fish were captured from the bank. Despite relatively low epibenthic densities, Chironomidae larvae were a prominent and preferred food item in the diet of all fish species, followed by Ephemeroptera in some (0þ Barbus barbus (L.), 0þ and 1þ Barbatula barbatula (L.), 1þ and !2þ Cottus gobio (L.)) and Simuliidae in others (0þ and !1 þ G. gobio, 0þ Leuciscus cephalus (L.), 0þ and 1þ Phoxinus phoxinus (L.)), with Trichoptera larvae, Asellus sp., Ostracoda and Annelida as tertiary prey taxa. The relatively consistent diel patterns observed suggest that data from single 24-h cycles in late summer can be representative of daily patterns. Day-time only surveys of European riverine fishes may give unreliable estimates of mean fish population size and abundance.