SUMMARY: With a glucose-limited chemostat culture of Azotobacter vinelandii iam 1078 the yield factor, Y X/S, for glucose increased from 0·3 to 0·12 following an increase in the dilution rate from 0·1 to 0·35 hr-1. However, even when growing at the faster rate the value of Y X/S still was extremely small compared to those found with other micro-organisms. This low yield value was not due to an incomplete oxidation of glucose, since most of the glucose carbon utilized (83 to 97%) could be accounted for as carbon dioxide, the percentage conversion depending on the dilution rate. The specific respiration rate (Q o2 ) of the growing culture was determined in situ and found to remain nearly constant over the range of dilution rates. However, the value of Y X/S diminished progressively from 0·12 to 0·03 when the dissolved oxygen concentration in a chemostat culture was increased from 3·3 to 5·7 p.p.m. The yield factor, Y ATP (evaluated by making certain assumptions) was much smaller than the value of 10 found with other microbial culture, even though the value of Y ATP increased appreciably with the increase of dilution rate. These results are discussed with reference to some of the known physiological characteristics of A. vinelandii.