Where introduced, largemouth bass (Micropterus salmoides) are known to have negative effects on aquatic ecosystems. In this study, we used historical museum collection records (1942–2006) and contemporary (2009) collections to assess changes in the distribution of largemouth bass in the lower Boise River (southwestern Idaho). We also examined the stomach contents of largemouth bass collected during the contemporary survey to determine diet. Seventy-four large-mouth bass (45–137 mm TL) were represented in 13 historical collections from 5 lower Boise River locations. During autumn 2009, we sampled 8 sites in the lower Boise River for largemouth bass. Sixty-one largemouth bass (range 55–156 mm TL; = 84 mm) were captured from 5 sites downstream of a 4-m-high diversion dam. Largemouth bass were absent from all sites upstream of the dam. Our contemporary collection data extends the known distribution of largemouth bass 7.2 river km upstream. The long-term persistence (without recent stocking) of largemouth bass in the lower Boise River indicates that the fish may be spawning in the river and/or entering the system from external sources. Eighty percent of the largemouth bass collected in 2009 were <100 mm TL. Analysis of largemouth bass stomach contents revealed that aquatic insects (40%), crayfish (37%), and small-bodied fishes (11%) comprised much of the diet. Our study confirms that the largemouth bass has successfully established in the lower Boise River and that the species is piscivorous at small sizes (<100 mm TL). These findings suggest that largemouth bass could have a negative impact on native fishes in the lower Boise River.