Despite the fact that the disease called Crayfish Plague has been known in Europe since the mid-19th century, there is no good description of the gross pathology. Nybelin (1931) indicates that the gross signs may be quite variable. Although no outbreaks of crayfish plague have been identified in the United Kingdom so far, the lack of information on the gross pathology has made it difficult to be certain that it has not been involved in some crayfish mortalities. For this reason, a culture of A. astaci was used to infect native crayfish, Austrapotamoebius pallipes. The crayfish were exposed to infection by placing actively growing agar cultures in their holding tanks. At 10-12°C mortalities commenced 58 days after exposure, and thereafter the exposed animals died rapidly: all showed similar gross external signs. Examination of the light coloured ventral surfaces showed numerous small, irregularly-shaped, dark brown flecks on the surface of the exoskeleton. Under a stereo-microscope, these flecks were seen to be heavily melanised areas, often joined together with thin lines which were clearly fungal hyphae. Many of these hyphae were also surrounded by a thin strip of melanisation. All surfaces of all parts of the animals were, in fact, affected, but the darker dorsal surfaces tended to obscure the signs of infection. Larger areas of melanisation were visible, particularly in the areas of joints of the body and of the limbs, which in a few cases were accompanied by perforations through the exoskeleton to the tissues beneath. None of a control group of unexposed animals died, nor did they show the exoskeletal features described.