'*C-labeled ribosomal ribonucleic acid (rRNA) was prepared from Azoto bacter chroococcum NCIB 8002, Azotobacter paspali 8A, Azomonas agilis NCIB 8636, Azomonas insignis WR 30, Beuerinckia indica NCIB 8712, and Azospirillum brasilense ATCC 29145. These rRNA's were hybridized under stringent condi- tions with filter-fixed deoxyribonucleic acid from a great variety of gram-negative bacteria. Each hybrid was described by: (i) the temperature at which 50% of the hybrid was denatured, and (ii) the percent rRNA binding (amount in micrograms of rRNA duplexed to 100 pg of deoxyribonucleic acid). These data were used to construct rRNA similarity maps. The following conclusions could be drawn concerning rRNA cistron similarities. (i) Bacterial genera with free-living, aerobic, nitrogen-fixing members are very diverse and belong to different rRNA superfam- ilies. The present family Azotobacteriaceae is not a biological unit, and its status as a family is highly questionable. (ii) Azotobacter chroococcum, Azoto bacter vinelandii, Azoto bacter beijerinckii, Azoto bacter paspali, Azoto bacter miscel- lum, Azotobacter armeniae, and Azotobacter nigricans belong in the genus Azotobacter. Any synonymy of these names remains to be determined. Azomonas agilis, Azomonas insignis, and Azomonas macrocytogenes constitute indepen- dent branches, which are about equidistant from Azotobacter and section I of Pseudomonas as presented in Bergey 's Manual of Determinative Bacteriology, 8th ed. Xanthomonas, Alteromonas vaga, and Alteromonas communis are lo- cated in the same rRNA superfamily. (iii) The genus Beijerinckia appears to be rather heterogeneous. Its closest relatives appear to be Xantho bacter autotro- phicus, "Mycobacterium" flavum, "Pseudomonas" azotocolligans, "Pseudomo- nas" diminuta, the authentic rhodopseudomonads, and some other organisms. These organisms belong in the same rRNA superfamily as Azospirillum, Agro- bacterium, Rhizo bium, Aceto bacter, Glucono bacter, and Zymomonas. (iv) Derxia belongs in still another rRNA superfamily , together with Chromobacterium, Janthinobacterium, the Pseudomonas acidovorans and Pseudomonas solana- cearum groups, Alcalienes, and a few other taxa. (v) The following organisms were generically misnamed: "Azomonas insignis" ATCC 12523, "Mycobacte- rium" flavum 301, "Pseudomonas" azotocolligans ATCC 12417, "Pseudomonas" diminuta CCEB 513, and "Rhodopseudomonas" gelatinosa (all strains exam- ined).