A pot experiment was carried out to study the efficiency of Bacillus polymyxa, Pseudomonas fluorescens and/or Streptomyces aureofaciens on biodegradation of the pesticide diazinon in sterilized and non-sterilized soil, either non-cultivated or cultivated. The obtained results showed that disappearance rate of diazinon was higher in the non-sterilized soil than in the sterilized one. Inoculation of soil with mixture of Bacillus polymyxa + Streptomyces aureofaciens and Pseudomonas fluorescens + Streptomyces aureofaciens exhibited higher dissipation rate of diazinon in either sterilized or non-sterilized soil, as compared with the soil inoculated with each microorganism individually. When the soil was cultivated with tomato plants and inoculation was made, biodegradation of diazinon increased, compared to the uncultivated and inoculated soil. The metabolites produced from biodegradation of diazinon by the tested microorganisms were 1,3-dimethy-2-nitrobenzene, diethylphosphate, 2-isopropyl-6-methyl-4-hydroxypyrimidine (IMHP) and 2-(1-hydroxy-1-methyl)-6-methyl-4 (IH)-pyrimidine (HMMP). Key words: Diazinon, biodegradation , B. polymyxa, P. fluorescens and S. aureofaciens.