A biological dosimeter has been developed for use in aquatic environments. This method is based on the sensitivity of a DNA repair‐deficient strain of Escherichia coli (CSR06) to ultraviolet (UV) radiation. The dosimeter permits evaluation of the penetration of biologically active UV radiation within a water column, reflecting the potential effect of exposure over selected time intervals. With the use of various filters, the biological dosimeter can discriminate between the effects of UV‐B (280–320 nm) and UV‐A (320–400 nm) or other selected portions of the solar UV spectrum. During springtime ozone depletion over the Antarctic in 1988, a general relationship was observed between stratospheric ozone concentration and the contribution of incident solar UV‐B radiation to lethality of dosimeter cells. The use of dosimeters within the water column indicated that significant amounts of UV‐B can be transmitted to a depth of 10 m and biological effects of UV could be detected to 20 and 30 m. Biological dosimeters may provide a means of standardizing in‐water UV measurements across all types of aquatic habitats and at any geographical location.