Use of transgenic crops, including those expressing the insecticidal Cry protein from Bt, is increasing at a rapid rate in worldwide.  Field and laboratory studies of transgenic Bt crops have been carried out to detect the persistence and activity of the Cry protein in soil and its effect on soil microorganisms to assess their risks to environment.  However, there were few studies that evaluate the seasonal effects of Bt rice on rhizosphere soil microbial communities compared to those of insecticides commonly applied in paddy soil for the control of lepidopteran insects.  In this study, seasonal effects of transgenic rice expressing the Cry1Ab insecticidal protein active against lepidoperan pests and the insecticide triazophos [3-(o,o-diethyl)-1-phenyl thiophosphoryl-1,2,4-triazol] on soil enzyme activities and microbial communities were compared under field conditions.  During a 2-year field study, rhizosphere soil samples of transgenic-Bt rice (Bt), non-Bt parental rice (Ck) and non-Bt parental rice with triazophos (Ckp) applied were taken at four stages in the rice developmental cycle: seedling, booting, heading and maturing.  Microbial processes were investigated by measuring different biochemical activities including those involved in C and P cycling.  Denaturing gradient gel electrophoresis (DGGE) and terminal-restriction fragment length polymorphism (T-RFLP) analyses were used to compare rhizosphere microbial compositions.  Some occasional and inconsistent effects of the application of triazophos on the bacterial composition in the rhizosphere soil of rice plant were found at the booting and heading stages as compared with that of transgenic-Bt rice.  There were no statistically significant differences (P>0.05) in phosphatase activity, dehydrogenase activity, respiration, methanogenesis or fungal community composition in rhizosphere soil between Bt, Ck and Ckp over the rice cropping cycle.  However, seasonal variations in the selected enzyme activities and microbial community composition in the rhizosphere soil of Bt, Ck and Ckp were clearly detected.  These results suggested that the changes in rhizophere soil microbial community composition associated with the crop growth stage overweighed the application of triazophos and the cry1Ab gene transformation.  KMD1 (Bt) rice expressing the cry1Ab gene had no measurable adverse effect on the key microbial processes or microbial community composition in rhizophere soil over 2 years of rice cropping.