The biotransformation of octylphenol polyethoxylate surfactant (Triton X-100) and bacterial communities in soil microcosms was investigated. The soil microcosms were designed to simulate real sites of bioremediation. The soil used in this study was contaminated with pesticides and alkylphenol polyethoxylate surfactants over a long period of time. The nitrogen source, (NH4)2SO4 and the mineral salt basal solution were added to the microcosm and the water content was adjusted to 50% with distilled water. The microcosms were aerated using an aeration system with an air flow rate of 0.3 l min(-1). The exogenous bacterial strain Pseudomonas sp. SH4 was added to the microcosms to increase the rate of biotransformation of the Triton X-100. The number of microorganisms and the theoretical extent of formation of carbon dioxide were estimated to evaluate the biotransformation of Triton X-100 in the microcosms. The analytical results revealed that the exogenous bacteria could increase the rate of transformation of Triton X-100 by approximately 50%. Aeration of the microcosm increased the biotransformation of Triton X-100 by 45%. Bacterial count of 6.8x10(10) MPN g(-1) of soil was achieved in the M5 microcosm. Analysis of the bacterial community by 16S rDNA sequences revealed that Pseudomonas sp. SH4 could dominate all the microcosms to which it was added as an exogenous bacteria. The quantity of the indigenous bacterial strains Stenotrophomonas maltophilia, Stenotrophomonas sp. and clone 4-70 were also enhanced in the microcosms by the aeration, and the addition of carbon and nitrogen source.