56 samples, including influent, primary effluent, secondary effluent and final effluent wastewater from two Spanish municipal wastewater treatment plants (WWTPs), were analyzed to assess both the occurrence and behavior of natural radioactivity during 12 sampling campaigns carried out over the period 2007-2010. Influent and final effluent wastewaters were sampled by taking into account the hydraulic residence time within the WWTP. A wide range of gross alpha activities (15-129 mBq/L) and gross beta activities (477-983 mBq/L) in liquid samples were obtained. A correlation analysis between radioactivity in liquid samples and the performance characteristics of the WWTPs was performed. The results in liquid samples showed that gross beta activities were not influenced by treatment in the studied WWTPs. However, gross alpha activities behave differently and an increase was detected in the effluent values compared with influent wastewater. This behavior was due to the increase in the total dissolved uranium produced during secondary treatment. The results indicate that the radiological characteristics of the effluents do not present a significant radiological risk and make them suitable for future applications.