Host plant genetics of N2 fixation in the cowpea (Vigna unguiculata (L.) Walp.) — Rhizobium symbiosis was investigated in the field with a mixed strain inoculum and in the greenhouse with a single strain inoculum. Five cowpea genotypes, including ‘H-Brown Crowder’ and ‘L-Bush Purple Hull’, were used to generate populations for genetic studies. Diallel analysis for general combining ability (GCA), specific combining ability (SCA) and reciprocal effects was carried out for the N2 fixation variables nitrogenase activity, nodule number, nodule weight, and top dry weight, in the field study. Generation mean analysis and heritability estimates were performed on the greenhouse population which included P1 (‘H-Brown Crowder’), P2 (‘L-Bush Purple Hull’), F1, F1′. F2, BC1, and BC2. Positive and significant correlations were observed between nitrogenase activity and both nodule weight and nodule number. Correlation coefficients between top dry weight and the other 3 variables were not significant. Estimates of SCA were highly significant for all variables except top dry weight, while those of GCA were significant only for nodule weight. Generation mean analysis revealed that additive gene action was more prominent than dominance and interallelic gene action for nodule number and nitrogenase activity, while the opposite was true for nodule weight and top dry weight. Narrow sense heritability estimates were moderately high for nodule number (0.55) and nitrogenase activity (0.62), and low for nodule weight (0.39) and top dry weight (0.17).