Sixteen isozymes, extracted from eleven tissues of healihy Hyriopsis cumingii Lea mussels and plague-diseased mussels respectively,were analysed with vertical slab polyacrylamide gel electrophoresis. The isozymes referred above were alcohol dehydrogenase (ADH), α-phosphoglycerol dehydrogenase (α-GPDH), lactate dehydrogenase(LDH). malate dehydrogenase (MDH). malic enzyme(ME), isocitrate dehydrogenase (IDH),6-phosphogluconate dehydrogenase(6PGD), glucose-6-phosphate dehydrogenase (G6PD), catalase, peroxidase (POD), cytochrome oxidase, glutamatepyruvate transaminase (GPT), alkaline phosphatase (AKP), esterase (EST), amylase and adenosine triphosphatease (ATPase). The isozymic phenotypes, tissue distributions, relative activities and mobile characteristics of the sixteen isozymes were determined, and the possible relationships between the isozyme pattern and gene expression were discussed. In comparison with the healthy mussels, the zymograms and activities of EST and α-GPDH in diseased mussels were evidently disordered. It provided the biochemical evidence that the mechanism of plague pathogenesis was closely related to the lipid metabolic block in digestive system of the diseased mussels. The pathological changes of the two isozymes in digestive system were considered to be an auxiliary biochemical diagnosis marker in the early stage of Hyriopsis cumingii Plague.