Effective root nodules formed onLotus pedunculatusbyRhizobium lotiNZP2037 andBradyrhizobiumsp. (Lotus) CC814s were examined by light and electron microscopy. NZP2037 nodules were larger than CC814s nodules as a result of a much thicker nodule cortex. The smaller CC814s nodules had a much larger infected zone and, consequently, larger volumes of infected cells, peribacteroid space, and host cytosol per nodule. As well, CC814s nodules demonstrated a significantly higher number of bacteria which typically contained prominent polar bodies and beta-polyhydroxybutyrate (PHB) granules. The bacteria of NZP2037 nodules lacked the polar bodies, but PHB deposits were occasionally observed. A correlated physiological study in which acetylene reduction was used to estimate nitrogen fixation showed CC814s nodules to be more active than NZP2037 nodules (Pankhurst, C. E., and D. B. Layzell. 1984. Physiol. Plant. 62(3) : 404–409.). This measured difference in nitrogen-fixing activity was considered due to the degree of infection and bacterial proliferation as well as differences in the carbon and nitrogen metabolism of the two nodule types. The significance of the structural observations and morphometric analyses and their relationship to differences in nitrogen fixation are discussed.