The use of insecticidal proteins known as Cry or Bt, either as biopesticides used in agriculture or as vector control or originating from commercial genetically modified crops (GM), is increasing rapidly. The fate of these proteins in the environment depends strongly on their adsorption on the organo–mineral complexes of soil. Environmental monitoring requires the quantification of the proteins and this entails their chemical extraction from soil. Three Cry proteins, Cry1Ac, Cry1C and Cry2A, present in commercial biopesticide formulations or synthesized by GM plants or both were studied. The adsorption of trace amounts of Cry proteins on over 40 types of soil with contrasting properties was measured in dilute suspension. After a short incubation the extraction yield was measured with a previously tested alkaline solution that contained surfactant and another protein. Each of the proteins had a strong affinity for soil. No soil property was observed to determine either the affinity for soil or the extraction yield. There was no simple relation between the affinity (assessed from the distribution coefficient, Kd) and the extraction yield, although there was a significant inverse relation (P < 0.05) for two of the proteins, Cry1Ac and Cry2A. The proteins differ in both their affinity for soil and their extraction yields. We conclude that these insecticidal proteins will be largely immobile in soil, but that routine environmental monitoring can give only semi-quantitative values for protein in soil.