Nine chlorinated phenols have been studied for their rate of biological degradation. The disappearance of the phenol has been observed by recording changes in ultraviolet absorption. The cleavage of the carbon-chlorine bond has been determined by observing the increase in the chlorine ion. There is some evidence that the decrease in concentration of the more biologically refractive compounds occurs by adsorption into the slime and that these compounds are not immediately degraded. When some sludge is mixed with agitation but without oxygen in the presence of more of the refractive compound, the concentration of the refractive compound in solution may increase sharply. The toxicity of these compounds toward fish shows a rough correlation with the ease of bacterial degradation. The toxicity of these compounds to fish is greatly reduced by an increase in the dissolved oxygen level.