In order to identify Rhizobium strains with higher biological N2 fixation capability in cowpea [Vigna unguiculata (L.) Walp.] cv. TC 9-6, 15 strains (7 slow growers and 8 fast growers) isolated from non-inoculated cowpea plants grown in the savannah soils of Monagas State, Venezuela, were evaluated. A completely randomized design with four replicates was used. Two control treatments were included, one non-inoculated and N-fertilized (KNO3 0.75 g L-1 of nutrient solution) and the other non-inoculated and non-fertilized. The plants were cultivated in plastic pots (4 kg of sterilized fine granite) and the strains in yeast extract and mannitol broth during 5 (fast growers) and 10 (slow growers) days. As inoculum, 2 mL of rhizobial suspension (1.07-1.22 x 109 active cells mL-1) were applied at sowing time and six days later. The plants were watered and harvested 45 days after planting. Nodule number and dry weight, shoot nitrogen concentration and content, shoot and root dry matter, and total dry weight were evaluated. Rhizobia inoculation was significant for all parameters evaluated. The highest shoot nitrogen content and total dry biomass identified the strains JV91, JV94, and JV101 as effective, i.e., with the highest potential for nitrogen fixation. Shoot nitrogen content correlated significantly with shoot dry weight (r = 0.85), total dry weight (r = 0.90), and nodule biomass (r = 0.72).