The agronomic importance of cultivated legumes has to a great extent directed the interest of microbiologists, plant physiologists, and soil scientists toward studies from diverse viewpoints of the symbiotic association of Rhizobium with its host plant. Initially, the goal was to identify the symbiont and establish stable, effective inocula for seed. Simultaneously, but to a lesser extent, the interaction of host plant and endophyte became the object of considerable biological and chemical investigation. During the period from 1888 to the mid 1930s there was only scant attention paid to non-leguminous, non-mycorrhizal associations. An increasing awareness of intriguing biological problems presented by these associations is due primarily to the pioneering efforts of G. Bond and his associates. The general topic has been the subject of many recent reviews and books (Schaede 1962; McKee 1962; Bond 1963, 1967 b ; Stewart 1966). It is my intention in this paper to review some of the biological and ecological aspects of the symbiotic associations in non-legumes as they relate to the process of biological nitrogen fixation. The scope of nodulation among non-leguminous plants Bond has prepared an excellent survey of the extent of nodule-bearing genera of angiosperms (Bond 1967 b ), listing over 300 species representing thirteen genera. Approximately one-third of the species examined bear nodules. Since many of the species are widely distributed and thrive in many adverse environments, they are undoubtedly of considerable ecological importance as pioneering plants. It is often difficult in the field to remove the plant root system without tearing off the fragile nodule clusters; therefore, any estimations of the ubiquity of nodulation within a given genus may be greatly underestimated. As will be discussed below, the role of the nodulated non-leguminous angiosperms in nitrogen fixation is better documented than that of the mycorrhizal associations of gymnosperms.