The freshwater sculpin species Cottus gulosus and Cottus perplexus present a challenge to biologists because of their overlapping sympatric distributions, morphological similarity, and historical inconsistency regarding species limits and ranges. We applied linear morphometric analyses in conjunction with meristic and discrete character counts to 1) assess the validity of previously cited diagnostic characters, 2) test multivariate combinations of characters to elucidate novel diagnostic characters useful in distinguishing the 2 species, and 3) apply these diagnostic characters to identify previously unidentified specimens obtained from Washington streams, where the 2 species occur in sympatry. Our analysis of 270 Cottus specimens encompassing the range of both species uncovered little support for the diagnostic utility of previously cited characters, although significant differences between mean measurements for each group were detected in the proportion of individuals with palatine teeth and a median chin pore, as well as in mean mouth-width to body-width ratio. Additionally, we were unable to detect linear transformations of continuous measurements that could be used to reliably identify individuals of these 2 species. These results prevented us from confidently identifying the unknown Washington specimens and illustrate that multi-locus phylogenetic studies are needed to assess species boundaries for these morphologically similar fishes.