The fact that the most of the bacteria isolated from the soil have the ability to dissolve rock phosphates both in the soil and in the culture medium is well known. The microorganisms produce low molecular mass organic acids, which attack the phosphate structure and transform phosphorus from non-utilizable to the utilizable for the plants form. The objective of the present study is to establish the effect of the rock phosphate concentration in the liquid medium and its particle size. As well as

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... the duration of the bioconversion on the solubilization of Tunisian phosphorite by Erwinia sp. and Azotobacter sp. isolated from soil. It was established that the lower the concentration of the phosphate in the nutritive medium the greater is the conversion percentage of P 2 O 5 into soluble forms. Also Azotobacter sp. have higher ability to dissolve Tunisian phosphorite than Erwinia sp. A maximum degree 96.7 % of P 2 O 5 extraction has been obtained for the period of 12 days of incubation of Azotobacter sp. when in the medium was added 0.5 % w v-1 phosphate with particle size below 0.2 mm. Investigations with Erwinia sp. showed that a maximum extent of 76.6 % of the phosphate solubilization has been observed on the 6 th day of incubation at phosphorite concentration of 0.5 % w v-1. It was observed a negative correlation between the degree of P 2 O 5 extraction and the culture pH and a positive correlation between the titratable acidity and the quantity of phosphorus released.