ABSTRACTIt was found that earthworms reacted sensitively and uniformly to ultraviolet (UV) irradiation in the levels of macroscopy, light microscopy and electron microscopy. In order to learn the probable role of superoxide radical and lipoperoxide on the mechanism of UV‐induced tissue damage, we studied the correlation of superoxide dismutase (SOD) and lipoperoxide, which was expressed as malondialdehyde content, in the UV‐irradiated earthworms.The SOD activity, which was assayed by the inhibitory effect of nitro blue tetrazolium reduction in the xanthine‐xanthine oxidase system, was initially elevated but later decreased gradually. Since this decrease preceded the UV‐induced pathological changes in the earthworm, it was hypothesized that the superoxide radical, the substrate of SOD, may be involved in this system. Lipoperoxide, however, did not increase in the same