In October 1977 an investigation was initiated of the qualitative and quantitative composition of the fauna of the eelgrass (Zostera marina) beds at Roscoff. Samples were taken in 2 seagrass beds at different tidal levels in order to follow numerical changes in the course of the year. In March 1978 the area of study was struck by the oil slick of the tanker “Amoco Cadiz ”. For this reason sampling has been continued at the same frequency. The tabulated results show clearly that the oil slick had a profound but selective influence on the various animal groups; some of them disappeared while others were apparently unaffected. Rapid recovery of some species has taken place, but re-establishment of other species, particularly the filter feeders has not been observed. The very diverse amphipod fauna has disappeared, and has been replaced by a population ofPherusa fucicola andGammarus locusta; the latter was absent in the year before the oil disaster took place.