ABSTRACT: Microbial activity was determined in the upper 10 cm of sediments and in bottom water of the European continental shelf margin in the Celtic Sea (Goban Spur: 48* to 50* N, 10* to 14* W; 135 to 1680 m deep). Assays used were the measurement of bacterialabundance (determined by epifluorescence microscopy), bacterial production (by thymidine incorporation) and hydrolytic enzymatic activity (by cleavage of fluorogenic model substrates). The highest activity was always measured in the top horizon (upper 1cm layer) of the sediment, while in the water overlying the sediment about 1 to 5% of the sediment's surface value was found. The highest hydrolytic activity was mostly achieved by protease followed by esterase, chitinase, beta-glucosidase, andalpha-glucosidase. Most tested activities in the sediment declined with increasing depth down the continental slope, but the steepness of decline was different for the various substrates. While protease and chitinase showed nearly no depth relation, therewas a strong response recorded for the 2 glycosidases. This leads to shifting ratios between the activities, indicating that not only microbial growth and general decomposition activity but also the type of reactions involved in the decompositionprocesses change with depth. Our observations may reflect differences in the microbial population structure as well as availability and biodegradability of specific compounds at the sea floor, as a result of successive losses of easily degradablesubstances from particulate organic matter on its way from the epipelagic zone to the sediment.