In the Seto Inland Sea in Japan harmful red tides have broken out year after year causing a lot of coastal environment problems such as degradation of scenery and damage to fish farming. To develop a plankton control ship equipped with a UV irradiation system, laboratory and field experiments were undertaken to investigate the effects of UV treatment on the harmful red tide algae. To clarify the relationship between UV dose at 254 nm and inhibition of the algal growth, several UV doses were examined by adjusting the distance between samples and UV lamp under different exposure times. Laboratory experiments were carried out for three red tide algae, Heterosigma akashiwo, Chattonella marina and Gymnodinium mikimotoi. A comparison was made on the numbers of moving organisms incubated for several days at 20 /spl deg/C and 5000 lux after treatments by UV radiation. It was found that while UV treatments inhibited algal growth for all species, H. akashiwo was inhibited most and C. marina least. The relation between UV radiation intensity (x) and exposure time to kill the cells (y) was expressed as y = ax/sup -b/ for all tested species. To clarify the effect of UV irradiation on red tide organisms sampled from a red tide in the Ariake Sea, Kyushu, field experiments were carried out using batch-process experimental equipment with five UV lamps. A 40-second UV treatment with the experimental equipment immediately destroyed the cells or stopped activity in about 80% of the Chattonella spp. That dominated the other algae. These results showed the possibility of reducing harmful red tide organisms by the UV treatment system.