There is a general belief that high levels of arsenic are found in waters of the northern states of Mexico. In order to test this hypothesis, research was conducted with the objective of determining arsenic levels in waters of the San Pedro River located in the state of Chihuahua, Mexico. From October 2005 to August 2006 on a monthly basis, water samples were collected at five points along the San Pedro River. Three repetitions were collected at each point resulting in a total of 165 water samples. Total arsenic concentration was L -1 detected in an Inductively Coupled Plasma-Optical Emission Spectrometer (ICPPOE) model 2100 by Perkin Elmer. The arsenic concentrations in waters of the San Pedro River ranged from 0.0110 mg L -1 to 0.587 mg L -1 with a mean of 0.104 mg L -1 ± 0.234 mg. Most samples were over the maximum permissible value established as Mexican and international standards. A quadratic projection was observed between the basic points through the fifth point; these results might be influenced by the decrease of flow and the decrease of the drainage level. These results confirm the hypothesis that waters in northern Mexico contain high levels of arsenic that potentially might be harmful to the whole ecosystem and being potentially dangerous to human development and health.