Aims Ecological risk assessments have paid increasing attention to the field release of transgenic Bt crops.  Some studies have reported that Bt protein could enter the soil ecosystem through root exudates, crop residues, crop decomposition and pollen.  Protein from transgenic Bt crops is a potential hazard for soil non-target organisms and biodiversity, and this may affect soil ecological function and soil nutrients.  This study aims to evaluate the soil ecological risk of transgenic Bt cotton.Methods A pot experiment with two types of transgenic Bt cotton, 'BtXincai1' and 'BtJi668', was conducted to study the effects of transgenic Bt cotton planting on rhizosphere soil microorganisms.  The plant was carefully removed from the pot at seedling, budding, flowering, bolling and boll-opening stages.  Rhizosphere soil approximately 0-5 mm in diameter was separated from bulk soil by gently shaking the root system.  Samples were frozen at |20 ? after sieving for subsequent Bt protein analyses, or stored at 4 ? for microorganism analyses, which were begun within a day of sampling.  Bt protein was analyzed by ELISA BOX.  The populations of three main microorganism groups and some functional groups were measured by cultivation.Important findings The content of Bt protein exuded by different transgenic Bt cottons and the degradation rate of Bt protein in different rhizosphere soils were different.  Compared with the controls, transgenic Bt cottons seemed to stimulate the reproduction of rhizosphere soil bacteria and fungi.  However, no significant effects on the populations of actinomycete, aerobic nitrogen fixing bacteria and kalium-solubilizing bacteria were observed.  Transgenic Bt cottons increased the population of ammonifying bacteria and decreased that of inorganic phosphorus-solubilizing bacteria significantly at seedling and flowering stages, whereas they increased the population of aerobic cellulose-decomposing bacteria and decreased that of organic phosphorus-solubilizing bacteria significantly only at the flowering stage.  'BtJi668' also enhanced the population of aerobic cellulose-decomposing bacteria significantly at the seedling stage.  The abundance of aerobic cellulose-decomposing bacteria and organic and inorganic phosphorus-solubilizing bacteria in transgenic Bt cottons rhizosphere soils changed.  Although the total population of functional groups in transgenic Bt cottons rhizosphere soils was higher than in control cultivars, community diversity (H) and evenness (J) decreased and dominance concentration (C) increased.  The change range of diversity indexes and population of functional groups in transgenic Bt cottons rhizosphere soils at the flowering stage was higher than at the seedling stage.  (Please note, article text is in Chinese.)