Although the toxic effect of a metal such as chromium on the normal B.O.D. of domestic sewage has been discussed before, sufficient information was desired in order to make a comparison of the oxygen utilization of the sewage without the chromium salts with those of increasing concentrations of chromium salts in terms of p.p.m. of dissolved oxygen depletions. The toxicity of both hexavalent and trivalent chromium has been observed in this experiment. The trivalent form showed the more consistent inhibition as the chromium concentration was increased. The hexavalent chromium reached a plateau in its effect on the B.O.D. of sewage when its concentration was 3.0 p.p.m., and further increases to 10 p.p.m. had no significant effect on the inhibition.