The Biological Reserve of Una, located in the south of Bahia, is a region with v ery little information about the composition, diversity and spatial distribution of the species. The objective of the present study was to know the composition and structure of the fish assembly of the streams of Biological Reserve of Una – REBIO. Four sampl es were taken at sev en points distributed in six streams, one being outside the reserve limits, between February 2011 and March 2012. The specimens were captured with trawls of the picaré and puçá type. In the field, the specimens were anesthetized, fixed in 10% formaldehyde and placed in 70% alcohol. The species were identified with specialized bibliography. The specimens that remained in doubt were sent to specialists from the Federal University of Rio Grande do Sul (UFRGS) and Pontifical Catholic University of Rio Grande do Sul (PUCRS) for identification. Then the material was deposited in the scientific collection of the State University of Santa Cruz (UESC). The occurrence constancy index, Shannon Diversity Index, Margalef Wealth and the Pielou Equity index were applied. In the ecological indexes, the ABC curv es and the MDS ordering method were calculated. A similarity matrix was calculated from the Bray Curtis index. W statistic consisting of a numerical summary of the ABC curve was applied. The t - test was used to verify the lev el of significance at 9 5%. A total of 5549 individuals belonging to 4 orders, 9 families and 22 species were collected, of which 8 had indefinite taxonomic status, which confirms that the region is little explored from the point of view of the ichthyofauna. The results indicate the dominance of opportunistic species and demonstrate the influence of environmental conserv ation on the composition of the fish congregation. The lack of studies in the region and human occupation refl ect the presence of undescribed species and the risk of extinction and the creation of a conservation strategy.