A comparison of the structure and organization of nodular tissues and bacteroids of peanut and cowpea induced by Rhizobium sp. strain 32H1 was madc 4 to 5 weeks after inoculation when nitrogenase activity reaches the peak. Observations revealed major differences that may have a role in the different rates of nitrogen fixation shown by the two species. All cell types in cowpca nodules were larger than those of peanut. The inner cortex of cowpea had an ‘endodermis-like’ layer of cells which was absent in peanut. All cells in the bactcroidai zone of peanut were infected but in cowpca many remained free of bactcroids. The bacteroid containing cells of peanut were isodianietrical and uniform in size with a central vacuole and a nucleus surrounded by tightly arranged bactcroids enclosed singly in peribacteroidal membrane sacs. Such cells in cowpea were mostly elongated with a nucleus and one or more vacuoles. The bacteroids within cowpea cells were arranged without any particular order with more space for host cellular material. They were mostly present singly in peribacteroidal membrane sacs which sometimes fused to enclose more than one bactcroid. The hosts seem to play the dominant role in the differentiation of nodular tissue and the morphogenesis of bacteroids in symbiotic systems induced by the same strain of Rhizobium.