To evaluate the potential effects of genetically engineered (transgenic) plants on soil ecosystems, litterbags containing leaves of non-engineered (parental) and transgenic tobacco plants were buried in field plots. The transgenic tobacco plants were genetically engineered to constitutively produce proteinase inhibitor I, a protein with insecticidal activity. The litterbag contents and surrounding soil, as well as soil from control plots without litterbags were sampled over a 5-month period at 2- to 4- week intervals and assayed for proteinase inhibitor concentration, litter decomposition rates, carbon and nitrogen content, microbial respiration rates and population levels of nematodes, protozoa and microarthropods. The proteinase inhibitor concentration in the transgenic plant litter after 57 days was 0.5% of the day 0 value and was not detectable on subsequent sample days. Although the carbon content of the transgenic plant litter was comparable to that of the parental plant litter on day 0, it became significantly lower over the course of the experiment. In addition, nematode population in the soil surrounding the transgenic plant litterbags were greater than those in the soil surrounding parental plant litterbags and had a different trophic group composition, including a significantly higher ratio of fungal feeding nematodes to bacterial feeding nematodes on sample day 57. In contrast, Collembola populations in the soil surrounding the transgenic plant litterbags were significantly lower than in the soil surrounding parental plant litterbags. Our results demonstrated that under field conditions proteinase inhibitor remained immunologically active in buried transgenic plant litter for at least 57 days and that decomposing parental and transgenic plant litter differed in quality (carbon content) and in the response of exposed soil organisms (Collembola and nematodes).