Peptidoglycan (PG) was isolated from Bifidobacterium thermophilum by TCA extracting , and its weight was 8. 4 % of dry weight of bacterium. The peptidoglycan contained 24. 2 % glycan , 3. 2 % fatty acid and 2. 4 % protein. The concentrations of Gly , Glu, Lys and Ala, the special amino acid of PG, were 157.2, 102.0, 173.8 and 1 061.1μmol?g -1 , respectively. After chemical compositionofPGwasanalyzed,thebiologicalactivitiesofPGwereexaminedtoo. EffectsofPG,whichhadbeendilutedto 0.1% and 1 % by special buffer and broken up by ultrasonic waves , on the activity of Phenoloxidase (PO) , Superoxide Dismutase (SOD) , Alkaline phosphatase (ALP) , Acid phosphatease (ACP) and Lysozyme (LSZ) of the serum of penaeid shrimp , Penaeus japonicus were studied. The results showed that the activity of PO and LSZ had been markedly enhanced and it could maintain in 96 h , the activity of SOD had been enhanced properly and it would decline back in 72 h , after ip PG. The effects of PG on the activity of ACP and ALP were similar , the enzyme activity was mildly increased in 96h. Except Penaeus japonicus , Paralichthys olivaceus were used as experiment animal too. PG could enhance the activity of SOD of fish serum significantly , but this activity was obviously different as time extending and it fell to the original value after 72 h passed. And different concentrations of PG had different effects. The results indicate that PG can be used as immunostimulant in order to enhance the non2specific immune activity of aquatic shrimps and fishes.