Several populations of the green peach aphid, Myzus persicae (Sulzer), were collected in the most important Italian peach-growing areas. In these populations, collected as primary colonies or as secondary ones, total esterase content and acetylcholinesterase sensitivity towards pirimicarb was assessed using biochemical assays. Most of the individuals analysed generally exhibited an elevated esterase content, and in some cases acetylcholinesterase insensitivity. The implications of resistance levels and distribution on M persicae control strategies are discussed.