Abstract. AimThis study aimed to verify if the water characteristics of Apodi/Mossoró River basin aquatic environments are mainly determined by a longitudinal gradient or its rain regime.MethodsThrough one year quarterly sample expeditions were carried (August/2007 to May/2008), in 24 sampling stations. In each place, water samples were collected and direct parameters were measured. Concentrations of nutrients, biochemical oxygen demand and thermotolerant coliforms were determined. For ordination of the sampling stations and in order to find patterns in water quality was conducted a Principal Components Analysis.ResultsThe results showed that points inside urban agglomerates had depreciated water quality, with high concentrations of nutrients. In general, it was not possible to define a longitudinal gradient for the studied variables, probably due to anthropogenic interferences along the basin. Otherwise, the effect of rainy and dry seasons were marked, resulting in a grouping of the sampling stations during the dry months, mainly due to the raised values of nutrients and biochemical oxygen demand.ConclusionIt could be concluded that in the rainy period a homogenizer effect occurred, not being verified the zonation of the hydrographic basin aquatic environments. The anthropogenic pressures probably contributed for the absence of a longitudinal gradient, since they generate disorders which modify the aquatic environments.