Nonnative species (NNS) pose one of the most common threats to the conservation of biodiversity. Given the difficulty in measuring and documenting the specific impacts of introduced species on native species, indices of condition may provide an easy and inexpensive alternative. The aim of this study was to understand the impact of NNS on some common and endemic freshwater fishes, examining their relative conditions in the absence (allopatry) and presence (sympatry) of nonnative fish species from 3 different regions in the western part of Turkey. Data from 19 sites, a total of 1672 fish of 11 cyprinid species, were used for calculation of condition indices. Relative condition was significantly higher in allopatry than sympatry for Capoeta sieboldii, Gobio gobio, Ladigesocypris ghigii, Leucaspius delineatus, and Phoxinus phoxinus; however, it was higher in sympatry only for Rhodeus amarus, whereas Barbus tauricus, Cyprinus carpio, Rutilus rutilus, Squalius fellowesii, and S. pursakensis showed no significant differences in relative condition between allopatry and sympatry. Results suggest that there was a possible impact of NNS on native species and that relative condition would provide an easy and useful alternative to get a preliminary evaluation of the impact of nonnative fish species as long as sampling follows proper methodology.