Analysis of mass‐length relationships of 10 populations of Mediterranean barbel from three basins with different environmental conditions showed that there were significant differences (ANCOVA, P<0∣05) in relationships between sampling sites, which could imply differences in environmental conditions. Oxygen concentration and riparian cover were found to be the environmental variables that accounted for most variation (83%) in parameter a of the mass‐length relationship between populations. Thus, mass‐length relationships of barbels can be used to assess environmental conditions in southwestern European streams.