The physical microhabitat requirements of freshwater pearl mussels Margaritifera margaritifera (L.) are now well known. Freshwater pearl mussels live buried or partly buried in clean, mixed stable substrate in fast-flowing unpolluted streams and rivers and subsist by inhaling and filtering water for the minute organic particles on which they feed. In 2002, a chance finding by a local crofter of live mussels in Shetland led to the discovery of a reproductively viable mussel population in peat dominated substrate in predominantly dry fen habitat. The physical habitat characteristics of the unusual Shetland site are contrasted and compared with conventional M. margaritifera habitat. The fen habitat and peat substrate used by the mussels are so atypical of that favoured by M. margaritifera elsewhere, that this is believed to constitute the first substantiated record of mussels using such a habitat.