Information available on the 1988 Chrysochromulina polylepis bloom in Scandinavian coastal waters is evaluated. Special attention is paid to the early bloom stage and the sequence of both environmental and physiological mechanisms leading to the bloom, and a tentative, integrated timing schema is given of causes which led to the observed events. Environmental conditions favourable for Chrysochromulina polylepis are considered to have resulted from : a) an exceptionally high runoff of nitrogen-rich water in winter and early spring; b) strong mixing of the water column, immediately followed by a long period of vertical stability and stratification; c) a diatom bloom 30 % larger than usual, peaking around 20 March; and d) a second diatom bloom in mid April, largely dominated by Skeletonema costatum. These events combined to cause the euphotic layer to become silicate-exhausted, phosphorus-poor and fairly nitrogen-rich. The lack of silicate prevented diatom growth. Weak turbulence and strong stratification favoured growth of non-siliceous, motile nanoplankton. C. polylepis is believed to have outcompeted co-existing algal species by producing a toxin acting first as a grazer repellent, while its cell density was still below 10(4) individuals.1(-1). At the conjectured location of first appearance of the bloom (or at one of several locations), in the Skagerrak near Gullmar Fjord, the grazing repellent forced a shift of grazing to other algae around the latter part of April. With increased population density of C. polylepis (10(6) cells.l(-1)), the toxin then became sufficiently concentrated to affect directly not only grazers, but all other organisms. From late April, the C. polylepis population increased free from grazing and other losses. In conditions of light and nutrient sufficiency, a high growth rate (corresponding to 0.8 div.d-1) was achieved. Actively growing cells were buoyant and remained in the upper part of the water column. With increasing population density (10(7) cells.l(-1)), nutrients became exhausted and self-shading decreased available light. By mid May, the growth rate had decreased to zero, and the population started to become senescent. C. polylepis deteriorated in physiological state, perhaps partly because of self-poisoning, and this led to sinking. No further growth occurred, but cell densities showed higher values (several 10(7) cells.l(-1), with maximum density between 5 a 10 m) because distribution became patchy both horizontally and vertically. At the end of May a significant part of the population had died, and cell leakage produced an increase in dissolved organic matter. The resultant lower turbidity and regenerated nutrients then allowed other flagellates and some diatoms to resume growth. The remaining C. polylepis cells sunk to the pycnocline (15-20 m), which acted as a barrier and greatly retarded this sinking, by which means the cells became concentrated, and produced the highest reported cell densities at the pycnocline. It is assumed that the bloom later sedimented. Since relevant environmental conditions for the initiation of the bloom occurred over a very large area (75.10(3) km2) and the dominant currents were not altogether consistent with the spreading of the bloom, polygenesis of the bloom should be considered a possibility. To what extent long-term global change and eutrophication might have contributed to this exceptional bloom remains unclear. Since the production of the toxin seems to be favoured by phosphorus deficiency, however, a large input of nitrogen in combination with a reduction in the phosphorus loading is believed to have contributed indirectly to the bloom, by changing the nutrient status of the coastal waters from being nitrogen- to phosphorus-limited.