The vulnerability to UV-B radiation inAcanthocyclops robustuswas analyzed in the laboratory. The LD50for adult individuals, determined by mortality one day after UV-B exposure was 40 μ Wcm−2for 4 h exposure. A significant increase in the mortality ofA. robustusadults was observed four days after exposure to UV-B irradiation. As a consequence, the estimated lethal dose at four days post-exposure was 20% less than that estimated two days after the exposure. These results indicate that because there is significant latent mortality during the post-irradiation period, accurate assessment of mortality due to UV-B exposure can only be made four days after the irradiation. Copepodite IV life stage individuals were much more sensitive to UV-B exposure than were the adults.