Resumo. Abstract Aim The present paper approached some issues related to the construction and adaptation of the Reservoir Fish Assemblage Index (RFAI). Method the case study presents the step by step construction of the multimetric index adopted for the Tietê cascade reservoir system, and it is discussed the comparison between discrete and continuous scoring criteria. Results The main questions related to the adaptation of multimetric indexes to reservoir were synthesized as: i – terminology question; ii - representative fish sampling; iii – reference condition. The construction of the RFAI for the Tietê cascade reservoir system resulted in seven steps, and the continuous scoring criterion showed to increase the accuracy of the final index when compared to the discrete scoring especially for the most degraded environment. Conclusion Biological criteria for the assessment of aquatic ecosystems are widely recognized and accepted by scientific community. The increasing value of water resource makes reservoirs important object of scientific and social interest, justifying the definition of proper tools for their assessment and monitoring. Among the available tools, the multimetric approach is one of the most popular. The observed results showed the validity of the approach also for artificial environments, expecting for its official incorporation in biomonitoring programs in Brazil.