The rapid worldwide expansion of genetically modified crops containing the Bt trait gives rise for concern as to the possible impact on non target species. These plants release insecticidal proteins, known as cry into soil during growth and decomposition of crop residues. The fate of the toxin is strongly influenced by its interactions with soil, in particular adsorption which modifies its biological properties, its mobility and its persistence. We describe three series of studies to investigate (i) the adsorption properties (ii) the mobility of the adsorbed protein and (iii) the decline of toxin in soil as a function of time and microbial activity. We conclude that the toxin will be strongly immobilised in soil and that microbial degradation does not determine the observed decline of toxin over periods of weeks and months.