This paper presents the modification of the Siret River Basin ichthyofauna under the influence of hydrotechnical facilities, through quantitative and qualitative monitoring of fish fauna of three dam lakes on the upper and middle basin of the river (Rogojeşti, Bucecea and Galbeni). On established species list, numerical and biomass abundance in each basin and on calculated the relative numeric and biomass abundance of fish species in investigated basins. The results were compared with literature data regarding the composition and structure of fish fauna of the Siret river and lakes during the period 1960-2000.