The periphyton is defined as a complex community of microorganisms (fungi, algae, bacteria, animals), which along with organic and inorganic particles is loosely or firmly adhered to a substrate submerged. Epilithic algae (bedrock) are organims found in environments of rivers and rapids, and other ecosystems. This paper aims to list the diversity of epilithic algae, estimate their relative abundance and frequency of occurrence during an annual cycle. In the main channel of the river Xingu was selected environment rapids of Boa Esperança and localities Arroz Cru. The epilíton was removed, properly identified and preserved in Transeau solution and subsequently analyzed in the laboratory. Were listed 132 species belonging to 78 genera. The total richness observed for Boa Esperança and Arroz Cru, in the months studied were 108 and 101 species respectively. Among all classes, Bacillariophyceae expressed greater contribution in both localities in terms of richness and abundance. The epilithic specie more abundant was Aulacoseira granulata (Ehrenberg) Simonsen (Bacillariophyceae), in the locations studied. In november/2006 and january/2007 (flood) occurred at higher densities mean: 133 and 124 ind.cm-2 in the riverine environment Boa Esperança. In the Arroz Cru was 107 and 90 ind.cm-2. The lowest mean density occurred in march/2007 (filled) with 26 ind.cm-2 in the riverine environment and Boa Esperança in august 2006 (flood) with 15 ind.cm-2 in Arroz Cru. The diversity of epilíton varied among seasons. The precipitation, wind, turbidity and nutrients, possibly influence the variations of abundance, richness and density.
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