Baltic herring (Clupea harengus L.) spawning ground surveys carried out in the northern Baltic proper have shown that, locally, eggs die in very high frequencies (up to 100%). These high mortalities have been shown to be connected to different kind of spawning substratum alge, predominantly filamentous brown algae. Laboratory experiments carried out in 1984,1987 and 1988 have shown the following algal species to have a significant negative impact on the survival of Baltic herring eggs: Pilayella littoralis (Ectocarpus siliculosus, Eudesme virescens and Rhodomela confervoides. The species of the genera Pilayella, Ectocorpus and Rhodomela displayed the most pronounced negative effects on egg survival in the experiments carried out in 1987 and 1988.The cause of this negative effect is still unknown.