Earlier research have shown that the haptophycean Phaeocystis pouchetii is toxic and that the excretion of toxins takes place at all phases of vegetative growth. There is thus a slow buildup of toxins over time in surface waters during a Phaeocystis bloom. Illustrating this by using examples from field situations and experiments we show that there are, from a ecosystem point of view, three phases in a Phaeocystis bloom: I. The coexistence phase where other phytoplankters and animals can share the biota with, but is influenced by Phaeocystis toxins; II. The mortality phase where all other species die out and/or escape and III. The "blank water" phase where plant and animal biomass approaches zero. Several of the effects of the toxin in question have consequences for commercial fisheries. The toxin may influence the feeding behaviour of cod larvae, or they may even experience mass mortality, food intake in aquacultured cage kept fishes may be reduced during spring, and we also claim that the cod spring fisheries along the coast of North Norway is brought to an end because buildup of toxins in the surface waters forces the fishes to migrate to deeper outlying waters. The scenarios we sketc illustrate that Phaeocystis is probably the single most important species in northern temperate areas that influences all trophic levels. It is food for herbivorous copepods and fish larvae, but it can also act harmful during periods of toxin buildup. Interesting is also, taken this in view, that Phaeocystis abundance varies between years, and that toxin production per unit Phaeocystis biomass increases with irradiance. By simple model studies we illustrate how variations in initial stock size, irradiance and dispersion rate may produce large between years effects on the ecosystem inhabitants.