The microbial degradation of phenol by pure and mixed cultures of Pseudomonas putida was studied in batch, phenol-stat, and continuous culture systems. In the continuous culture runs, both steady state and transient experiments were performed. From these experiments, a model for the kinetic behavior of the organisms was evolved and an analysis performed on the stability and dynamic behavior of pure and mixed cultures. The results indicate that it should be possible to achieve phenol removal from wastewaters down to levels of 1-2 ppm in a single state system. However, because of the effect of substrate inhibition on kinetic behavior of the microorganisms, long lasting transients can occur. The transient behavior of such systems cannot be solely determined from mumax or Ks parameters, but must include a consideration of the transient size and response characteristic of the organism.