The effect of strain specificity, various culturing conditions, and incubation periods on the growth and efficiency of dinitrogen fixation by A . lipoferum and A . brasilense, grown in semisolid S? malate medium was investigated. The properties of the particular strain markedly affected the proliferation as well as the nitrogen p i n s of Awspzrillum in either growth condition. In A . lipoferum strain 1244 and A . brasilense strain 1771 the multiplication rates were much higher, the counts reaching 7 x lo6 and 9 x 106 cells/ ml aft,er 15 days in cage of 50/300 ml growth condition, associated with high nitrogen gains reaching 99.8 and 114.3 mgl-l, while in A . lipoferum strain 8670 and A . b.rasilense strain 5677 the counts were 6 x 105 and 2 x lo6 cells/ml after both the same period and growth condition, associated with low nitrogcn gains, reaching 62.5 and 69.6 mgl-l, i. e., the amounts of nitrogen fixed is usually linked t o cell proliferation. The microbial activity in both nitrogen fixation and oxidation of carbonaceous material is directly reflected on the efficiency of dinitrogen fixation. I n all treatments the efficiency of N,-fixation generally increased markedly with time. I n A . lipoferum strains 1244 and 8670 the efficiency of N,-fixation was 118.5 and 107.4 in case of 50/100 ml growth condition after 20 days, whilc in A . brasilense strains 1771 and 5677 the corresponding figures were 126.5 and 115.4. The response of the available oxygen was linked to the growth conditions, accordingly, the efficiency of N,-fixation increased by 4.4% & 8.8% in case of A . lipoferum strains 1244 and 8 670, and 11.0% 8: 9.8% in case of A . brasilense strains 1771 and 5677 as a result of increasing the surface area: depth ratio by altering the volume of medium in the experimental flasks.