and phosalone, and the activity of three enzymatic systems reported to be involved in resistance. Eleven field populations and a susceptible strain used as a reference were tested using a bioassay consisting in the topical application of a diagnostic concentration on post-diapausing larvae. The enzymatic activity of mixed-function oxidases (MFO), glutathione-S-transferases (GST) and esterases (EST) was measured in post-diapausing larvae and adults. A significant decrease in the efficacy of the organophosphates was observed for all field populations, though the decrease was smaller in the case of chlorpyrifos-ethyl. No differences between sexes were detected. In post-diapausing larvae, the activity of the three enzymatic systems was higher in all the field populations than in the susceptible one. The possible implication of EST in codling moth insecticide resistance is reported for the first time in European field populations. In adults, only MFO and GST were implicated. Codling moth resistance to organophosphates in Spanish populations must be taken into account in the implementation of anti-resistance strategies.