The alimentary tract content composition of 106 European minnow (Phoxinus phoxinus, 25-75 mm TL) and 49 gudgeon (Gobio lozanoi, 31-123 mm TL) captured in August 1996 in the river Larraun (Northern Spain) was investigated in order to analyse their diet composition and interspecific diet overlap. Both species fed mainly on Chironomidae larvae, Trichoptera larvae and terrestrial invertebrates, although trophic diversity was significantly higher for European minnows. The diet composition and the feeding strategy plots revealed a similar ontogenetic shift of the relative contribution or these prey items to a more pronounced generalist feeding strategy as fish of both species grew larger. Additionally, comparisons between diet composition and the benthic macroinvertebrate community revealed that P. phoxinus and G. lozanoi refused Caenidae, showing a preference for Chironomidae and Psychomyidae. The simplified Morisita index proved that the diet overlap between the two species was indeed very high and significant. We discuss how morphological constraints - i.e. mouth size - , prey handling costs and habitat partioning could be responsible for the observed inter- and intra-specific (size-related) changes in diet preferences. However, we suggest that the combination of differential microhabitat use, diel feeding rhythms and diet plasticity could minimize the interspecific competition, allowing co-occurrence of these Cyprinidae species in relatively high densities in this reach of the river Larraun.