abstract A study is made of the dynamic interaction, through the soil, between two parallel infinite shear walls placed on rigid foundations. The steady-state response of both structures for a vertically incident SH wave is obtained and compared with the corresponding values resulting from consideration of only one structure. It is found that the additional interaction effects caused by the presence of a second structure are especially important at low frequencies and in the neighborhood of the fixed-base natural frequencies of the second structure. For high frequencies it is sufficient to consider the interaction between each structure and the soil, ignoring the presence of other structures.