Variation in diel activity and seasonality of populations which comprise fish assemblages hás been associated to mechanisms to allow coexistence in environments of limited resources. In reservor, especially those with few large river contribution, the envioronmental modification caused by the impoundment, results in the development of new populations adapted to the new lentic environment, relatively closed, since the stream hardly are used by as way for fish migration; in such cases, fishes show higher probability to develop mechanisms for occupy the available niches as temporal variation. The study aims to determine the composition of the icthyofauna and their variation according to diel and seasonal changes in Lajes reservoir, RJ, an oligotrophic environment biult in the sea mountains tropical rainforest, by damming small rivers. Adittionally, environmental variables influences on fish occurrence were assessed. Fishes were caught by gill nets, between September 2001 and August 2002, with the nets being revised each 3 hours throughtout a 24-hour period. Fishes relative abundance and environmental variables were compaared among season and diel period (day, sunset, night and sunrise) by using univariate non-parametric tests and patterns were determined by multivariate analyses. A total of 696 individuals, weighting 78172,06g, comprising 19 species, were caught in the 64 sampling. The most numerous species in decreasing order were Astyanax parahybae, Oligosarus hepsetus, Trachelyopterus striatulus, Metynnis maculatus and Geophagus brasiliensis, representing more than 58% of the total catches, and contributing each species more than 10% of number, and 35% of ocurrence in samples. These abundant species, with exception for M. maculatus, showed signficant (p<0.05) diel variation in relative abundance. Species of nocturnal/crepuscular activity were the Characiformes A. parahybae and O. hepsetus and the Siluriformes T. striatulus, while G. brasiliensis showed activity during the day/crepuscular. Seasonally, significant (p<0,05) changes were found for M. maculatus only, peaking in summer and paucity in winter. The examined environmental variables (conductivity, water level, dissolved oxygen, pH, rainfall, depth, temperature and transparency) did not show difference throught the diel cycle, although all of them, with exception of transparency, showed significant seasonal variation (p<0.05). Multivariate ordination and clustering showed two fish assemblages, one associated to the day and another to the night, but no seasonal pattern was detected; these multivariate analyses also showed a well defined seasonal patter and no diel patter for the environmental variables. The lowest diversity (H ́-Shannon-Wiener), richness (D-Margalef), number of species and individuals occurred during the day in all seasons. Diel variation strategy, rather than seasonal or environmental variables, was the machanisms used by abundant fish species to coexit in Lajes reservoir, which most part of the community presenting nocturnal activity.
