Reproduction is the mechanism by which species generates new offspring to perpetuate itself. Variation in rainfall and river level had been identified as the most important factors in freshwater fish reproduction in the tropics. Dams and reservoirs change the seasonality of this factors so, fish assemblage must develop new strategies to survive. Our objective was to define the period and sites of reproduction of fish assemblage in the Porce River, modified by two reservoirs in cascade (Porce II and Porce III dams) using the Reproductive Activity Index (R.A.I.) and define if the local rains are related to. Until now we have done four samplings that corresponded to drought periods (September ¿ October 2011 and February ¿ March 2012) and rainy periods (November ¿ December 2011 and May-June 2012) during years 2011-2012. We sampled twenty four sites (reservoirs Porce II and Porce III, the river channel between those reservoirs and downstream Porce III reservoir and, tributary streams to Porce III reservoir). Fishes were caught with gillnets and electrofishing. Fishes were identified, weighted, measured in its standard length and gonadal maturity stage was defined according to the scale of five stages. Reproduction activity of the assemblage was evaluated throughout (R.A.I) and its relation to rains was defined by Spearman correlation index. The highest R.A.I. was observed in the River Channel and in the Porce II reservoir. Reproductive activity of the fish assembly in the two reservoirs and in the Porce River was permanent and it was independent of climate seasonality; although the flow of river Porce to reservoirs is variable, the fish seem to find favorable conditions for its reproduction in these systems independently of seasonal climate. In the streams, there is an increase in R.A.I. during the periods of rains.