A graphical method for determining the 5-day biochemical oxygen demand (BOD5) for a body of water is described. In this bioassay, students collect a sample of water from a designated site, transport it to the laboratory, and evaluate the amount of oxygen consumed by naturally occurring bacteria during a 5-day incubation period. An accuracy check, prepared by spiking a portion of the water with a glucose-glutamic acid solution, is used to evaluate the student's technique. This graphical treatment requires a larger number of samples than the standard method described in the literature, but it is not prone to errors caused by the quality of dilution water, which can invalidate measurements made with the standard method. (Contains 3 figures.)