Short-distance (i.e., < 100 km) introductions of diminutive fish species are often not well documented but may have important ecological consequences. Prickly sculpin (Cottus asper), which are native to lowland habitats of the Pacific Northwest, have been introduced in some mountain lakes of western Washington State. The ecology of six introduced populations of prickly sculpin was investigated through daytime minnow trapping, diet analysis, and age and growth analysis. Results of minnow trapping indicated prickly sculpin were abundant in each lake (Nisqually River lakes, mean = 11.3 individuals/trap; Dry Bed Lakes in Satsop River basin, mean = 1.9 individuals/trap). Prickly sculpin diet in the Nisqually River lakes was composed primarily of micro-crustaceans (92% by number of all prey items; dominated by copepods and cladocerans), chironomids, mollusks, and sculpin In the Dry Bed Lakes, the diet was composed primarily of caddisflies, oligochaetes, and micro-crustaceans. In native lentic habitats, prickly sculpin primarily consume macroinvertebrates with the rare occurrence of micro-crustaceans in the diet. Similar to native populations, prickly sculpin in the mountain lakes reached sizes greater than 150 mm total length; however, their growth rates were slower. In conclusion, prickly sculpin were common in the mountain lakes, consumed a variety of prey types and sizes, and grew to a relatively large size; therefore, this species may have important effects on the ecosystem of these lakes. However, additional assessment of their native and introduced distribution and ecology is needed to better understand their potential as an invasive species.