A study of the relationship of the raw wastewater BOD5:cBOD5 ratio was conducted at three wastewater treatment plants across the United States. Gwinnett County, Georgia; King County, Washington; and Littleton-Englewood, Colorado conducted a 3-month study using side-by-side samplers. The purpose of the study was to determine if cleaning wetted sampler parts routinely affected the ratio. One of the samplers was cleaned twice a week and the other was not cleaned. The results of the study show that cleaning the wetted parts of the sampler has a small but statistically insignificant impact on the BOD¬5:cBOD5 ratio. The study showed other surprising relationships in the ratio and other aspects of BOD5 and cBOD5, including the impact of wastewater temperature and sampler type.