Biochemical oxygen demand has been used for many years as a parameter in the study of water pollution. It is useful in the design and operation of waste treatment facilities, in the evaluation of the effects of wastes on streams, and in the determination of water quality. In the past, many authorities have considered it the most important single parameter available. This particular study was undertaken to fill gaps in the available information on the precision and accuracy of the oxygen demand determinations. Through simultaneous analysis of an identical sample, it is hoped that the common variables of the methods can be evaluated.