1. At 37 stations in the English Channel and the southern North Sea concentrations of four brominated phenols in Lanice conchilega were determined using ECD equipped capillary gas chromatography: 2,4-dibromophenol, 2,6-dibromo-4-methylphenol, 2,4,6-tribromophenol, 3,5-dibromo-4-hydroxybenzaldehyde.2. Concentrations in worms of the southern North Sea were generally below 1 mu g/g wet wt.3. Levels were slightly raised in worms of sheltered shores, those of 3,5-dibromo-4-hydroxybenzaldehyde were increased in subtidal populations.4. Concentrations in L. conchilega of Brittany were considerably higher than those in worms of the Frisian Islands, North Sea; they deviated from the latter by factors of 3, 16, 4, 4, respectively.5. The reasons for the conspicuous differences are hitherto unknown; three explanations are suggested.