It is now widely documented that reduced ozone will result in increased levels of ultraviolet (UV) radiation, especially UV-B (280-320nm), incident at the surface of the earth [Watson 1988 ozone trends, Anderson Toohey Brune 1991 free radicals, Schoeberl Hartmann 1991, Frederick Alberts 1991, WMO 1991, Madronich Chapter 1993, Kerr McElroy 1993], and there is considerable and increasing evidence that these higher levels of UV-B radiation may be detrimental to various forms of marine life in the upper layers of the ocean. With respect to aquatic ecosystems, we also know that this biologically-damaging mid-ultraviolet radiation can penetrate to ecologically-significant depths in marine and freshwater systems [Jerlov 1950, Lenoble 1956 Revue d'Optique, Ray Smith Baker PCPB 1979, Ray Smith Baker science 1980, Ray Smith Baker 1981 clearest natural, Kirk Hargreaves Morris 1994]. This knowledge, plus the dramatic decline in stratospheric ozone over the Antarctic continent each spring, now known to be caused by anthropogenically released chemicals [Solomon progress 1990, Booth Lucas Morrow Weiler Penhale 1994], has resulted in increased UV-environmental research and a number of summary reports. The United Nations Environmental Program (UNEP) has provided recent updates with respect to the effects of ozone depletion on aquatic ecosystems ( Hader, Worrest, Kumar in UNEP 1989, 1991, Hader, Worrest, Kumar and Smith UNEP 1994) and the Scientific Committee on Problems of the Environment (SCOPE) has provided [SCOPE 1992] a summary of the effects of increased UV radiation on biological systems. SCOPE has also reported [SCOPE 1993] on the effects of increased UV on the biosphere. In addition, several books have recently been published reviewing various aspects of environmental UV photobiology [Young Bjorn Moan Nultsch plenum book 1993], UV effects on humans, animals and plants [Tevini 1993 lewis], the biological effects of UV radiation in Antarctica [Weiler Penhale Geophysical 1994], and UV research in freshwater ecosystems [Williamson Zagarese 1994 chuck]. Several other reviews are relevant [NAS 1984, Caldwell camp 1986, Worrest overview 1986, NOAA 1987, Ray Smith 1989 PCPB, Ray Smith Baker 1989 stratospheric oceanography, Voytek 1990, Hader 1993, Acevedo Nolan 1993, Holm-Hansen effects 1993, Vincent Roy 1993, Biggs Joyner 1994 Springer, Williamson Zagarese 1994 chuck, Karentz 1994 tolerance, Cullen Neale 1993, Cullen Neale 1994 depletion]. As Hader et al. have summarized [UNEP 1989, UNEP 1991 effects], ``UV-B radiation in aquatic systems: 1) affects adaptive strategies (e.g., motility, orientation); 2) impairs important physiological functions (e.g., photosynthesis and enzymatic reactions); and 3) threatens marine organisms during their developmental stages (e.g., the young of finfish, shrimp larvae, crab larvae).'' Possible consequences to aquatic systems include: reduced biomass production; changes in species composition and biodiversity; and alterations of aquatic ecosystems and biogeochemical cycles associated with the above changes. Within the past four years, our knowledge with respect to the environmental effects of ozone-related increased levels of UV-B has increased significantly, and numerous efforts have been directed toward process-oriented studies of UV responses in plants and animals. Consensus is building toward the view that current levels of UV play a major role as an ecological determinant, influencing both survival and distribution, and are thus deserving of increased study independent of ozone-related UV-B increases. This review outlines U.S. research subsequent to 1991 and emphasizes studies concerned with phytoplankton.