Abstract: Potato growers continue to rely on neonicotinoid insecticides, primarily imidacloprid and thiamethoxam, for Colorado potato beetle (Leptinotarsa decemlineata) control. While neonicotinoids remain highly effective, the consistent use of one class of insecticide further increases the risk of resistance development. Populations from across the United States were bioassayed for both imidacloprid and thiamethoxam resistance in 2004 and 2005. In addition, laboratory populations have been selected with imidacloprid and thiamethoxam to increase resistance to each compound. High levels of resistance were found outside of Long Island, NY, with beetles from commercial sites in Delaware and Massachusetts being greater than 200 fold and 60 fold resistant to imidacloprid, respectively, compared to a susceptible strain. For the first time, a population in the Great Lakes region showed elevated levels of resistance to imidacloprid (>20 fold). Resistance to imidacloprid was highly correlated with resistance to thiamethoxam, although resistance to thiamethoxam remains at a lower level than resistance to imidacloprid. Monitoring of resistance and cross resistance is a critical part of resistance management.