Manipulation of the host genotype has been proposed as a method of increasing biological nitrogen fixation by rhizobia in symbiosis with the peanut (Arachis hypogaea L.). The F₁ generation of a diallel cross of 10 South American cultivars was evaluated in the greenhouse in an analysis of gene action for traits related to nitrogen fixation. The parents represented five secondary centers of diversity and effects in the diallel model were partitioned into among- and within-center components. Variation of center effects was significant for several characters but was smaller in magnitude than within-center variation. Specific combining abilities were significant and accounted for more variability than general combining abilities for nodule number, nodule mass, specific nitrogenase activity, shoot weight, and total nitrogen, indicating non-additive types of gene action. Maternal effects were observed for the same characters. The parents with the highest general combining abilities (GCA's) for nitrogen fixation were both fastigiate types, while Virginia-type parents had generally low GCA's. Correlations between parental and GCA effects were nonsignificant for all traits, so simple evaluation of lines for nitrogen-fixing capacity may not identify superior parents for use in breeding programs.