In July 2014, the widely cultured pond crab (commonly called Chinese swimming crab) had a disease outbreak along the coasts of Tianjin and Hebei provinces, China, resulting in massive mortalities. Symptoms of two different illnesses appeared in the crabs. One was white muscle disease (WMD), with its symptoms being anorexia, slowed movement, and the muscles turning opaque white and finally emulsifying into a toothpaste-like consistency. Mortality among the crabs displaying WMD was 60%-70%. The other was milky disease (MD), which displayed as a softened carapace, cessation of feeding, slowed movement, and the muscles similarly turning opaque white but then becoming watery (‘milky') and finally shrinking; the coelomic fluid and the deteriorated tissue leaked from the body and had not coagulated. Mortality among the crabs displaying MD exceeded 90%. According to the characteristics of the epidemic, and histopathological and ultrastructural analyses, the supposed cause of WMD was infection with a microsporidian. The parasite was ellipsoid, measured (1544±250) nm, and had the following structure: pole filament (8 or 9 laps), spore wall [thickness (130±22) nm], cilium (length 120-500 nm), polaroplast [(726±200) nm], pole tube, anchoring disk, plasma membrane and nucleus. The MD outbreak was caused by sp., a monad, sized 8-10 μm, with an oval shape, and wrinkled and raised surface; its basic structure was a cell wall, multiple nuclei, cilium and other structures. Histopathological examination showed that the two kinds of diseased crabs had significantly different muscle lesions. The crabs associated with WMD had many microsporidia adhering to their muscle fibers, causing the fibers to break, collapse and necrosis, finally reducing them to muscle bundles. SEM observations showed that the muscle cells became fused and acquired a large number of vacuoles, while the mitochondria and endoplasmic reticulum disintegrated and disappeared. In comparison, the gap in the muscle bundles was bigger in cases of MD, in which numerous parasites occupied or adhered to the muscle fibers, resulting in the fibers rupture, disintegration, and resolvable necrosis. Electron microscopy also confirmed that the two different parasites could infect the same tissue in the same crab, and both parasites could enter the disintegrated cytoplasm after muscle cell fusion. Since the initial epidemic of these diseases, other occurrences of WMD and MD have tended to cause a higher level of mortality, causing concern as significant diseases currently effecting the crab-farming industry.