Earthworms are important soil organisms. Now a days many pesticides and chemical fertilizers are applied in the agricultural field. They are slowly eradicating earthworm population in soil. Thiacloprid is commonly used as insecticides in Tamilnadu. Hence in the present study was selected to observe toxicity of thiacloprid on biomass, gut microbial population and digestive enzymes activity of an Indian earthworm, L. mauritii. After exposure to thiacloprid low (1/10th of 96 h LC50 - 0.03 ppm kg−1) and high sublethal concentration (1/5th of 96 h LC50 - 0.03 ppm kg−1), bacterial and fungal population, digestive enzymes such ascellulase, α amylase, β amylase, protease and lipase level were decreased at 5th, 15th, and 30th day of experiment compared with control. The biomass of L. mauritii was also decreased upto 90 days. The results reported that thiacloprid insecticide severely inhibited the earthworm.