This research deals with the composition and structure of macrozoobenthic community of River Silis, Northern Sardinia. Samplings, carried out in different periods of the year, allowed to display the seasonal evolution of communities. Results showed a scarce diversification of taxa but a well structured community. The recolonization, after summer drought, was also observed. At the starting of this process the most resistent organisms were present, and increasing the flow, new taxa appeared raising notable values of density. Data, utilized to determine the water quality by means of E.B.I. method, show that class of River Silis varies from II-I in the upperstream to III-II in the downstream. These results point-out a situation of environmental disturbance due to urban wastes and summer drought.