Results of a study on organophosphate (OP) resistance carried out on 4 Italian field populations of Culex pipiens are reported. The A1, A4-B4 and/or A5-B5 nonspecific esterases and insensitive acetylcholinesterase (AChE) were detected in our samples. A2-B2 esterases previously recorded in Italy were not observed. The A4-B4 and/or A5-B5 esterases were first found in Italy where they are at present widespread. Both nonspecific esterases and insensitive AChE are involved in OP resistance, although the high level of OP resistance observed in the Padova population could be correlated with both a high frequency of insensitive AChE and A5-B5 esterases.