Bioaccumulation of five organochlorines pesticides [lindan, aldrin, heptachlor epoxide, 4,4'-dichloro-diphenyl-trichloroethane (DDT) and hexachlorobenzene (HCB)] by the rainbow trout was studied in a laboratory continuous flow system at concentration level more closing to that in natural environment (ca. 2 microg/l). Under such conditions, bioconcentration kinetics of all the pesticides by the rainbow trout was almost linearly increased during the exposure period of 20 days for tested chemicals, excepting for lindan. For lindan, approach of an asymptote in curve (wet weight basis) could be observed at the end of exposure. The concentration factors (CFs) of the five pesticides based on whole body weight were in the range of 1000-7000. The difference in CFs may be attributed to the octanol water partition coefficients (Kow), size and steric effects of the chemicals. The result showed that the bioconcentration kinetics was different from those steady states at relatively high concentration in short period. Rainbow trout was a suitable kind of fish to study the concentration of organic pollutants for it's rather low base value, sensitive and easy to be treatment. Difference profile between the water and rainbow trout indicated that measurement of total concentration of pesticides in water is insufficient to assess bioavailability and toxicity of the organic pollutants to organisms.