Rhizobial strains were isolated from root nodules of Vigna trilobata plants raised in soils of different districts of Andhra Pradesh, India. Among the 23 strains of rhizobia isolated, 6 strains were proved to be positive for phosphate solubilization. The strains were identified as Sinorhizobium sp. strain MRR101-KC428651, Agrobacterium tumefaciens strain MRR102-KC428652, Rhizobium sp. strain 103 -JX576499 ; Sinorhizobium kostiense strain MRR104-KC428653; Agrobacterium tumefaciens strain

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... C428654 and Rhizobium sp. strain MRR106-KC428655 after 16S rDNA sequencing. S. kostiense strain MRR104 was found to be better than strains of Agrobacterium tumefaciens and Rhizobium sp. in phosphate solubilization with maximum zone of solubilization (18mm) on standard Pikovskaya's medium and with maximum P 2 O 5 liberation of 510 µg/ml in liquid medium. The optimization for maximum phosphate solubilization was done by using different carbon and nitrogen sources. Glucose was preferred as carbon source by all the strains studied, with 10 fold increase in solubilization of phosphorous than other carbon sources. Phosphate solubilization increased with increase in concentration of glucose up to 3% in all the strains studied. Some strains preferred ammonium sulphate and others preferred nitrates as nitrogen source for phosphate solubilzation, indicating that strains of V. trilobata are adopting two different mechanisms for solubilization. Reduction in pH with increased phosphate solubilization efficiency was also observed among the strains, irrespective of the carbon sources tested. Strain S. kositense MRR104 was proved to the better strain with maximum libration of phosphorous along with maximum reduction in pH as 5.13. Sinorhizobium strains performed better than that of Rhizobium and Agrobacterium in solubilization of phosphorous.