This paper deals mainly with the non-symbiotic nitrogen-fixing bacteria other than B. Azotobacter and B. saccharobutyricus. The two latter have received a considerable amount of attention but comparatively little work has been done on the less active nitrogen -fixing bacteria of soils, especially in recent years. An endeavour has been made to isolate and stú.dy the aerobic bacteria which, in addition to B. Azotobacter, probably play an importan part in the fixation of elementary nitrogen in soils. In view of the fact that practically no work has been done on the nitrogen-fixing organisms of Egyptian . soils, a complete study of representative samples of the latter was also aimed at. A number of investigations which dealt with many of the organisms studied in the course of this work have been previously carried out. The following is a short review of the more important literature concerned with non-symbiotic nitrogen -fixing bacteria other than B. Azotobacter and the anaerobic forms. B. aerogenes and similar organisms (Aerobacter, B. aerobacter, B. acidi lactici, and B. pneumoniae) have several times been found to be capable of assimilating free nitrogen (15, 37, 42, L3, 39, 56), and in experiments in which they were cultivated along with other organisms results were also obtained which pointed to their probable nitrogen- fixing capacity (3, 14, 52). Other bacteria which may belong to this group have been found to fix nitrogen -(4, 31). In certain experiments strains of B. aerogenes isolated from soil produced no fixation of nitrogen (3, 56). Strains of B. cloacae which were isolated from soil have been found to be unable to fix nitrogen (56 B. ra_diobacter has proved to be a nitrogen- fixing organism (15, 16, 22, 23, 24, 37, 380 142, 43, 49, 56), although certain negative or doubtful findings have been recorded (3, 56, 61). Of the three strains of the B. lactis innocuum type isolated from soil, only two were found to be capable of fixing nitrogen (43, 49). Two strains of B. lactis viscosum from soils brought about a small fixation of nitrogen (37, 38, 43, 49) . A strain of B. prodigiosum from soil exhibited the capacity of assimilating nitrogen (37). Another positive result with the same organism, which, however, was obtained under doubtful conditions, has been recorded (25). B. fluorescens and related types have been found to fix nitrogen in certain experiments (1l, 22, 33), but not in others (37, L3, L.9). Other short rods which have been found to be capable of fixing nitrogen include B. turcosum (38, )42), B. chrysogloea (22, 1!2), B. tartaricum (42), B. Krakataui (28) , Pseud. leuconitrophilus (47), B. alcaligenes (11), B. aurentiacus (11),B. candicans (11), B. azophile (18), B. Truffauti (63) and B. lipsiense (t2), although the fixation recorded in the last case is doubtful. Mic. sulfurens (112) and a similar unidentified micrococcus (29) have shown the power to fix nitrogen A considerable number of sporing rods have been found to fix nitrogen. B. megatherium was able to fix nitrogen in certain experiments (21, 59, 60, 62), but negative results have also been obtained with this bacillus (56). B. Fllenbachensis has shown the capacity to fix nitrogen (21) but several negative results have also been obtained with this organism (20, 27, 55). Several other sporersin which the capacity to assimilate nitrogen has been demonstrated include B. mesentericus (3), B. asterosporus (8), B. tumescens (11), B. ruminatus (11), B. oxalaticus (t2) B. malabarensis (42), B. danicus (14)1, L5), and a similar organism to the latter (24) and Planobacillus nitrofigens (5). A large number of bacteria which have not been identified have been recorded as nitrogen -fixers (13, 29, 34, 36, 48, 53). Certain organisms have been recorded as having fixed nitrogen in impure culture (2, 50). A number of organisms have recently been described (10) which produce an apparent fixation in soil but practically all failed to do so in solution. The following organisms have been found to' be incapable of assimilating elementary nitrogen: B. coli (43, 49, 56), B. agreste (37), Mic. pyogenes (14.3, 49),B, subtilis (21, 42), although a fixation by thé latter (255) has been obtained as a result of an unsatisfactory experiment.