Cultivation of transgenic plants is debated worldwide.  Potential environmental risks have to be considered, before acceptance of expanding cultivation, despite the advantages of the use of fewer pesticides.  Here, the potential effects on soil ecosystems of transgenic plants have been studied.  As a model, genetically engineered cotton producing cowpea trypsin inhibitor (CpTI) has been used.  The degradation of CpTI in the rhizosphere of the transgenic CpTI+Bt (Bacillus thuringiensis) cotton cultivar SGK321 was assessed.  During plant development, concentrations of CpTI toxin in the rhizosphere were measured using an enzyme-linked immunosorbent assay (ELISA).  As the plants developed, the residue of CpTI in the rhizosphere increased, and reached a peak at topping stage (100 days after planting).  After this stage, the residue began to decrease, and was nil the following year (258 days after planting).  The conclusion is that genetically engineered cotton can safely be cultivated since no accumulation of substances released from the transgenic plants was persistent in the soil.