Abstract: The symbiotic interaction between rhizobia and legume roots is characterized by a high degree of specificity. Two varieties of cowpea were gamma irradiated as a one method to create genetic variation resulting in new varieties with better characteristics in nodulation and nitrogen fixation processes. Conjugation is the second method used in this study, a cell contact-dependent DNA transfer mechanism, which has served as elegant tool in the development of genetic engineering technology. The possibility of horizontal gene transfer to other rhizobia, revealed that it is necessary, in view of possibility of deliberate release of a variety of recombinant rhizobia into the environment for such agricultural purposes as improving nitrogen fixation. New recombinants revealed higher amounts of indole compounds from tryptophan above the mid-parents in two out of six transconjugants resulted from the cross between P1 x P3. Significant number of nodules were developed on the root system of V2-variety in M4 generation treated with 20 krad in response to inoculation with the parental strains (P2 and P3) and also in response to inoculation with triparental transconjugants (Tr4 and Tr5), above that developed on the plants fertilized with recommended dose of N. The results revealed the success of rhizobial strains and their recombinants to colonize and infect roots of cowpea, because of significant dry weight of nodules per plant which can be obtained in V1-variety treated with 20 krad in M4 generation inoculated with the parental strain (P3), above that on the plants fertilized with recommended dose of N. Total chlorophyll formation in V1-variety inoculated with di-parental transconjugants (DPM-Tr2 and DPM-Tr3) at all doses of gamma irradiation was significantly increase above that in the plants fertilized with recommended dose and the mid-parents, with the exception at 30 krad if compared with the mid-parents. Significant increase was resulted in fresh weight of pods developed per plant above the mid-parents in M3 generation of V1-variety at doses zero and 10 krad, in response to inoculation with di-parental transconjugant, DPM-Tr2. While, the same trend was also achieved above the full dose in M3 generation at 10 krad in response to inoculation with DPM-Tr2, DPM-Tr3, TPM-Tr4 and TPM-Tr5. The highest nitrogen content was appeared in the shoots of V1-variety at all doses of gamma irradiation in response to inoculation with diparental transconjugant (DPM-Tr2). However, V2-variety had the lowest nitrogen content in relation to the plants fertilized with recommended dose of nitrogen and to the mid-parents of rhizobial transconjugants. The genetic variability of grain-protein content appeared that V2-variety treated with 10 krad had significant increase in protein content above that in the plants fertilized with recommended dose of N among M3 and M4 generations, in response to inoculation with parental strains and most of their transconjugants. The same trend was also shown in M4 generation of V1-variety treated with 20 and 30 krad above the plants fertilized with recommended dose of nitrogen, in response to inoculation with di-parental transconjugants. All biochemical traits studied were more affected by biofertilization than the doses of gamma rays and the interaction between biofertilization x doses. This indicated that the significance of treatments was mainly due to inoculation and particularly to gamma irradiation and the interaction between both of them.