Harmful algal blooms present a growing problem for fisheries, coastal ecosystems, aquaculture and public health. A widespread belief suggests that these outbreaks are due to increasing coastal eutrophication. Other factors, such as global climate change (including ozone depletion and increasing UV radiation) may also affect algal blooms and successions by differential tolerance of species to environmental stress. We examined 152 species (206 strains) of cultured microalgae from 12 classes for the presence of UVA- and UVB-absorbing compounds. Cultures were grown under white fluorescent light with no supplementary UVA or UVB radiation. Tetrahydrofuran/methanol extracts of microalgae were examined for ratios of UV absorbance (280-390 nm) to chlorophyll a (665 nm). Three groups of species were found: those with low UV : chl a ratios (0.18 to 0.9, diatoms, green algae, cyanophytes, euglenophytes, eustigmatophytes, rhodophytes, some dinoflagellates, some prymnesiophytes), those with intermediate ratios (0.9 to 1.4, chrysophytes, some prasinophytes, some prymnesiophytes), and those with very high values (1.4 to 6.75, surface bloom-forming species of dinoflagellates, cryptomonads, prymnesiophytes and raphidophytes). HPLC analysis of bloom-forming raphidophytes and dinoflagellates showed suites of UV-absorbing mycosporine-like amino acids including mycosporine-glycine, asterina-330, shinorine, porphyra-334 and palythine. Local strains of the successful toxic bloom-forming dinoflagellate, Gymnodinium catenatum, also contained major quantities of unknown UV-absorbing compounds. Evidence suggests that the presence of UV \'suncreeens\' may allow bloom-forming species to dominate in UV-rich environments.