The freshwater pearl mussel Margaritifera margaritifera is an endangered species. To survive, the young mussels demand a fairly coarse bottom material containing an appropriate quantity of organic substrate to survive without suffocating. In order to estimate if a bottom is suitable for juvenile mussels, samples of the sediment are needed. To obtain samples a sampling device called the Mulv collector has been developed within this project. Bottom samples were collected from 18 stations in 4 south Swedish streams, Vramsån, Kling-storpsbäcken, Brönnestadsån and Bräkneån. No rejuvenation of the mussel was found at the stations containing more than 25% fine material.