The strong connection between ecology and morphology in fishes predicts that ecologically relevant morphological features will change in concert with changes in habitat use and feeding ecology even over relatively short timescales following species introduction. Here, this hypothesis is tested by examining the trophic morphology of specimens of the native Tessellated Darter, Etheostoma olmstedi, collected at Catatonk Creek, Candor, Tioga Co., NY, USA, between 1961 and 2002. The collections of E. olmstedi bracket a 1980 range expansion by the non-native Banded Darter, E. zonale. Examination of the E. olmstedi specimens revealed that the length of the buccal cavity relative to body size increased following the invasion of Catatonk Creek by E. zonale. An increase in buccal cavity length (BCL) is expected to result in higher suction feeding performance measured as volumetric flow rate of water through the mouth. Such an increase in BCL may enable E. olmstedi to exert greater suction forces on prey and as a result, post-invasion E. olmstedi may be able to capture more elusive prey or to take prey from habitats with smaller substrates than pre-invasion E. olmstedi. The functional and potential ecological consequences of the increase in BCL suggest that the morphological change may be linked to a shift by E. olmstedi to more marginal habitats following the invasion of E. zonale.