Abstract: The influences of transgenic Bt rice straw on anaerobic microbial populations and enzymatic activities in paddy soil were investigated by using the improved Hungate anaerobic technique, including anaerobic MPN (most probable number), roll tube and spectrophotometer methods. The results showed that the transgenic Bt rice straw could not only increase the number of hydrolytic-fermentative bacteria (P0.05) and decrease obviously that of denitrifying and methanogenic bacteria (P0.05), but also increase the population of anaerobic nitrogen-fixing bacteria significantly. The significant effect of transgenic Bt rice straw was observed on the activities of phosphatase and cellulase (P0.05), however, the activity of dehydrogenase was seriously inhibited ((P0.05)) in short time after returning transgenic rice straw into flooded soil.