Working, by WINKLER method for determination of D. O. of fresh water, collected at and next to Escola Superior de Agricultura "Luiz de Queiroz", of Universidade de S. Paulo, the A. has found that of the 17 determinations, which were made, 6 showed numbers lower than limits established by Sanitary Code of Estado de São Paulo, Brazil, respecting dissolved oxygen (D. O.). This shows that tested waters are unsafe by use as potable. RIDEAL-STEWART modification was also used for the same samples, with results again more disfavorables for fresh water. Biochemical oxygen demand (B. O. D.) moreover was determinated in same samples.