Extraordinarily high levels of chlorophyll a and high cell numbers were found in the Skagerrak and Kattegat areas in mid January. Fluorescence maxima were found at about 10 meters depth at several stations. The diatoms Skeletonema costatum, Pseudo-nitzschia delicatissima-group and Thalassiosira nordenskioeldii dominated the phytoplankton samples. P. delicatissima-group was found with cell numbers above its critical limit at Å17 and N14. Some clones of this genus can produce AST (Amnesic Shellfish Toxin). No AST have however been reported in Swedish waters. In the Baltic Sea the number of species and the cell numbers were low causing low chlorophyll a values which are normal for the season.