SUMMARYAssays on adult females and eggs ofPanonychus ulmi, on leaf discs of Myrobalan plum, were used to characterise and establish homogenous organophosphateresistant (OR) and susceptible strains. Reciprocal crosses of these strains, and assays on adult mites and eggs of the parent, F1and F2generations confirmed that resistance was controlled by a single major gene, the expression of which was dominant in the response to parathion, and incompletely dominant to vamidothion, dimethoate and demeton‐S‐methyl. Reproductive incompatibility, between strains, as inTetranychus urticae, was not found.Bioassays on larvae hatched from winter eggs, collected from several orchards in Kent, Essex and Somerset, were used to evaluate the spectrum of cross‐resistance to a range of 20 organophosphates (OPs). Together with assays on adult females, these tests showed 10‐ to 100‐fold resistance to most of the OPs used for several years including those noted above, but less than 10‐fold to azinphos compounds and to several recently‐introduced OPs, e. g. dialifos, triazophos, dioxathion. The resistance spectrum in strain OR and five field strains was similar, but some had higher levels to parathion and demeton‐S‐methyl, and others to dioxathion after 3–4 years' exposure. Resistance levels to individual OPs may be enhanced by specific selection. Strains resistant to OPs showed only 2‐ to 5‐fold resistance to two carbamates, and were fully