Bioassay data for a reference strain of Nasonovia ribisnigri (Mosely), exhibiting similar responses to proven susceptible strains of Myzus persicae (Sulzer) and Aphis gossypii Glover, were used to assess insecticide resistance in two suspected resistant strains and nine field strains of N. ribisnigri originating from lettuce in the UK. Results showed widespread but varied levels of resistance to pirimicarb, lower and also varied resistance to pyrethroids and organophosphates, and no significant differences in response to imidacloprid. In some strains, resistance was associated with an intensely-staining esterase band disclosed by polyacrylamide gel electrophoresis (PAGE). However, no direct relationship between esterases and resistance has yet been established. There was no biochemical evidence of an altered acetylcholinesterase contributing to pirimicarb resistance in these strains.