The phytoplankton samples from the Skagerrak station Å17 had the highest diversity of the plankton stations in the Western Sea in mid-March. Diatoms were the most frequent and the diatom Leptocylindrus danicus the most numerous species. At Släggö and at the Kattegat stations the species diversity was lower. At Släggö and at the Kattegat station Anholt E a minor bloom of the prymnesiophyte Chrysochromulina hirta was observed. The integrated chlorophyll a concentration (0-20 meters) was below average at Anholt E, but within average for this month at all of the other Western Sea stations. In the Baltic Sea a bloom of the diatom Skeletonema costatum was observed at BY2 in the Southern Baltic and at Ref. M1V1 in the Kalmar Sound. The integrated chlorophyll a values were within normal for this month at the Baltic sampling sites.