Sediment bioassays have usually been applied to sediments from sites close to known sources of pollution such as harbours or major industrial sites. In this study sediment bioassays were conducted on samples from 120 sites in the Kattegat and Skagerrak (North Sea) at water depths ranging from 4 to 682 m. Sediment toxicity to Daphnia magna, expressed as immobility after 48 h exposure to 16% sediment (wet weight) ranged from 0 to 95%. Sediment toxicty to Nitocra spinipes, expressed as the 96 h LC50 for sediment in water, ranged from 0.61 to 88% dry weight. The responses obtained in duplicate bioassay test vessels showed good correlation (r = 0.7–0.8), but toxicity to Daphnia was not correlated with toxicity to Nitocra. High sediment toxicity was not restricted to areas close to industrial sites and harbours, but was found also in open sea areas. Analysed sediment concentrations of contaminants (heavy metals, organochlorines and polyaromatic hydrocarbons) at 25 of these 120 sites were tested for correlation with sediment toxicity, but no correlations, or rather weak ones, were found. Further, the concentrations of single contaminants were usually below those, which are considered harmful to aquatic organisms (tentative sediment quality criteria). Therefore, none of the single analysed contaminants seems to cause the observed sediment toxicity. No improvement in correlation was achieved when using an additive model of joint toxicity between different groups of contaminants and the combined toxicity to Daphnia and Nitocra. Therefore, the cause of the sediment toxicity is so far unknown, and no hazard assessment based on the analysed contaminants was warranted. The average immobility of Daphnia after exposure to 16% sediment (wet weight) for 48 h was 29%, and the average 96 h LC50 for Nitocra exposed to various concentrations of sediment was 11.4% (dry weight) of sediment in dilution water. A tentative hazard assessment based on these average results from both bioassays suggested that sediment toxicity in the Kattegat and Skagerrak must be reduced by on average four times to approach a Predicted Effect Level (PEL) allowing 50% survival.