This study investigated the toxic effects of cadmium on viability, reactive oxygen species (ROS) production and non-specific esterase activity of Penaeus monodon haemocytes in vitro, using a flow cytometric assay. After 6 h in vitro exposure with 10(-9)-10(-3) M Cd(2+), cell viability, ROS production and esterase activity of haemocytes from P. monodon were determined. Results showed that at the lowest exposures (10(-9)-10(-6 )M), Cd(2+) induced no effect on cell viability, ROS production and esterase activity. At a higher level (10(-5) M) of exposure, production of ROS was stimulated while Cd(2+) had no effect on cell viability and esterase activity. At the two highest concentrations (10(-4) and 10(-3) M), Cd(2+) caused increased ROS production, cell death and inhibited esterase activity. These results showed a relationship between Cd(2+) exposure dose and its cytotoxicity on shrimp haemocytes. Cadmium was cytotoxic and immunotoxic for P. monodon haemocytes in vitro when the dose reached 10(-4) M. The study also suggested that flow cytometry could be used as a tool for cytotoxic research of aquatic contamination on shrimp.