Aims: The objective of this paper was to study the adaptation dynamics and biochemical response of Azospirillum lipoferum grown in a continuous culture at various environmental shifts. Methods and Results: The kinetics of A. lipoferum Sp 59b grown at steady states in a microaerobic chemostatic environment deviated from a typical Monod kinetics in both low and high dilution rates (D) due to several metabolic shifts that occurred in the microbial cell. When NH4Cl was exhausted (at low D), the microbial cell partitioned carbon flow in order to sustain growth, nitrogen fixation and assimilation processes (occurred via the glutamate synthase reaction). Increasing D the specific activities of the enzymes involved in the tricarboxylic acid cycle and the respiration rate were decreased. At transitory states, under optimal for nitrogen fixation dissolved oxygen (DO) concentrations, ammonium nitrogen negatively affected, besides nitrogen fixing activity, the bacterial growth. At sub-optimal for nitrogen fixation DO concentration (i.e. 1.56 mu M) and 0.1 g l(-1) NH4Cl in the fed medium, the activities of citrate synthase and succinate dehydrogenase were significantly reduced. Conclusions: Important shifts in both carbon and nitrogen metabolism occur in A. lipoferum grown in the presence of the ammonium nitrogen, while the boundaries of ammonium nitrogen concentration in which A. lipoferum can be adapted depend on the DO concentration in the growth environment. Significance and Impact of the Study: Studies on growth dynamics and physiology of A. lipoferum, grown in experimental model systems, can contribute to an efficient application of these bacteria as plant-growth-promoting-agents.