Age distribution of American eel (Anguilla rostrata) populations was compared at sites downstream from each of the first four dams inland from the mouth of the Shetucket/Thames River, CT., and at one site upstream of the fourth dam, during August, 1977. Ages were determined by interpretation of otoliths. The significant differences between age distribution of populations below (1 and 2) and above (3–5) the second dam revealed that dams did impede upstream movement of American eels in the Shetucket/Thames River. Additionally, length-weight relationships reported in other areas of the northeast, and in the entire Thames River Basin, were not significantly different from the pooled sample of American eel in the Shetucket/Thames River.