Abstract: The electronic microscope technique and the enzyme cyt ochemistry technique were used to studied the ultrastructure of the wall of the midgut of penaeus chinensis, and active site of phenoloxidasa (PO) and acid phosphatase (ACP) in the tissue cells of midgut well. Right midgut from penaeus chinensis were removed and making experiments using three methods: ①midgut tissue was cut into blocks of approximately 1mm 3, the blocks were fixed in 3% fresh glutaraldehyde solution and then were post-fixed in 1% osmic acid. Embeldded it in Epon821. Semit him sections of the midgut ware cut on the ULTRACUT E. Stained it with toluidine blue and examined under microscope. Ultrathin sections of the midgut were cut on the LKB-Nova ultramicrotome. ②The midgut tissue blocks were fixed in 2% glut araldehyde solutions, then the blocks cut into thin sections approximately 60nm and incubated in the ACP medium and post-fixed in 1% osmic acid, embedded it in Epon821 and cut it into ultrathin sections. ③the midgut tissue blocks of healt hy and virus-infectious penaeus chinensis were fixed in 1% Bakers formol calciu m-and cut it into thin sections approximately 100 nm, incubated it in the pheno l oxidase medius then deal the samples with common ultrasection technique. All abo ve ultrathin sections were observed under a Hitachi H-7000 electron microscope. The results showed: the epithelium of midgut includes light cells and da rk cells, mostly dark cells, both kinds of cells have the function of absorbing nutrient substances from the enterocolic cavity and transferring them to connect ive tissue. Cytochemistry reaction results showed: ACP positive reaction was sho wn in the rounded vesicles that are situated at the top of the light cells, also in the lysosomal bodies of both light and dark cells. The tops of the dark cell s were crowded with lots of ACP-negative zymogen granules that were mainly synt hesized and excreted by the dark cells. PO hardly have activity in health y wall of the midgut, while in the virus-infections penaeus chinensis, the PO- granules were found on the disassembled granular cells of the connective tissue and dissociate in the connective tissue. Besides maintaining the shapes of the epithelial cells and stoking nutriments, the connective itssue play role in the i mmune reaction of the penaeus chinensis.