In order to evaluate the effect of the the Clarin channel reopening on the fish community around the lagoons of the External Delta of the Magdalena river, fish samples were collected each month in 15 stations. This channel was opened with the purpose to recover the hydric equilibrium in the region and the mangrove forest and fauna. The fish samples were used to analyse the variations of their community structure between three subregions: Cienaga Grande of Santa Marta, Pajarales and Salamanca Occidental and, between the years (1995 and 1996) before and after the Clarin channel opening. A total of 4161 fishes of S9 especies were captured, and the salinity was found the most is (classification and significance test) showed that the differences of the community structure between zones were greater than between the two years compared. This reflect the higher degree of regional influence from the Magdalena river, than from the local Clarln channel. The year of study was characterized by lower values of salinity than years before in the area. This account from the abundance of the freshwater species found.