An inexpensive, easily deployed polymer film was used to measure the penetration of ultraviolet (UV) radiation in nearshore tropical water. Measurements were made to a depth of 25 m in Discovery Bay, Jamaica, with a UV-sensitive polymer film in which o-nitrobenzaldehyde is converted to o-nitrosobenzoic acid. The film is sensitive in the range of 400-300 nm. UV radiation ranged from 5 pmol quanta s-l m-2 at the surface to 0.05 at 25 m. The diffuse attenuation coefficient (k) varied between 2.6 and 0.09 m-l with an average of 0.33 m-l. Diurnal measurements of UV at 1 m showed the shape of the curve to be similar to that of surface photosynthetically active radiation, with a maximum at 1200 hours. Integrated measurements ofUV are useful in conjunction with the response of aquatic organisms to changes in this radiation.This paper describes the use of a film actinometer to measure penetration of ultraviolet (UV) radiation into clear marinc waters. The film has the advantage of being sensitive over almost the entire range of solar UV radiation reaching the earth's surface. This method offers a suitable way in which to make field measurements that may then be related to biological processes or used as descriptors of a particular water type.The measurement of solar radiation is of concern both to physicists interested in its inherent properties of penetration through the earth's atmosphere and waters and to biologists who study the effects of radiant energy on organic processes such as photosynthesis, development, and behavior. An important component of the solar spectrum is UV radiation which is subdivided somewhat arbitrarily into three groups: UV-A (400-320 nm), UV-B (320-280 nm), and UV-C (280-200 nm). UV is high energy radiation and therefore produces disproportionate biological effects relative to its 5% ' Current