Abstract: In China, there are many strains of cruc ian carp in different area. According to genetic theory, the chromosomal numbers and D NA contents of different species can reflect the relationships among species bec ause of invariableness of themselves. Generally, cellular DNA content was determined by Feulgen staining and micro-fl uorescence dense miter. However, its manipulation is lasting and complicated by this method. In present paper, using flow cytometry (FCM) the DNA contents of er ythrocytes from red crucian carp (Carassius auratus var. red),Pengze crucian carp (Carassius auratus var. Pengze)and gibel carp (Carassius auratus gib elio)were measured and comparisons between them were carried. Four live fishes from every strain were collected from Guanqiao experimental Station of Institut e of Hydrobiology, the Chinese Academy of Sciences. From every experimental fish 0.5mL of blood were collected, and erythrocytes were isolated by means of prec ipitation. Cock red cell was served as control. After raising and PI staining, D NA contents of erythrocytes were measured by flow cytometry (FCM, Beckman Coulte r).Measuring dates were treated and assayed by microcomputer. The DNA contents of erythrocytes from red crucian carp (Carassius auratus va r. red),pengze crucian carp (C. auratus var pengze) and allogynogenetic cru cian carp (C. arasstus gibelio)were measured and comparisons between them we re carried. Results indicated that DNA content of erythrocyte was 3.0 picograms (pg) in red crucian carp, 4.7 picograms(pg) in pengze crucian carp, and 4.8 pi cograms (pg) in allogynogenetic crucian carp. Obviously, DNA content of triploid pengze crucian carp was 1.57 times of diploid red crucian carp, and content of allogynogenetic crucian carp was 1.6 times of diploid red crucian carp. Ploidy investigation of red crucian carp, pengze crucian carp and allogynogenetic cruc ian carp was undertaken by chromosomal number counts of mitosis metaphase of kid ney cell. The result showed that chromosomal number of red crucian carp was dipl oidy (2n=100), chromosomal number of Pengze crucian carp was triploidy(3n=162) a nd chromosomal number of allogynogenetic crucian carp was from 156 to 162(3n=156—162). The research revealed that there was a significantly positive correlatio n between DNA content and ploidy. Use of flow cytometry technique as ploidy dis crimination of fish have some advantages (convenience, rapidity, credibility) an d made up some shortages of chromosomal analysis. It provided new methods and th eoretical basis for checking up of quality of fish. At the same time, it can also be used as ploidy discrimination of rare freshwater and marine and introduced polyploidy fish.