Two transgenic rice lines, KMD2 and Huachi B 6, both contain a synthetic cry1Ab gene from Bacillus thuringiensis (Bt) under the control of maize ubiquitin promoter, were used in the laboratory to examine the degradation of Cry1Ab toxin proteins from Bt transgenic rice in paddy soils. Ground leaves of KMD2 and whole leaf-blades of Huachi B6 were separately buried in each of three types of soil, i. e. pale paddy soil on quaternary red soil, blue clayey paddy soil and marine-fluvigenic yellow loamy paddy soil, and thereafter residues of Cry1Ab in the KMD2-leaf-mixed soils and Cry1Ab in the buried Huachi B 6 leaves were examined at an interval of 6-10 days. Cry1Ab in ground leaves of KMD2 degraded rapidly in all the soils during 36 days after treatment (DAT), particularly during the first six DAT, with the highest degradation rate in blue clayey paddy soil and the lowest in marine-fluvigenic yellow loamy paddy soil. During 6-36 DAT, the highest Cry1Ab residue was observed in marine-fluvig enic yellow loamy paddy soil, mediate in pale paddy soil on quaternary red soil, and the lowest in blue clayey paddy soil, with significant difference tested among soils at each of observation times; however, such difference decreased gradually as time proceeded thereafter, and became fairly little at 78 DAT. As the soil was flooded, the degradation of Cry1Ab in buried leaves of Huachi B 6 was significantly accelerated, and exhibited a similar pattern among all of the flooded soils. Such impacts of flooding occurred only in the first 12 DAT, and during most of the subsequent periods no significant difference could be observed in Cry1Ab residue between the flooded and unflooded treatments. Degradation kinetics of Cry1Ab toxin proteins were fitted to exponent equation in each treatment, and half life of the toxin protein was determined. Finally, impacts of micro-organism and organic matter content in soils on the degradation of Cry1Ab were discussed.