An investigation of 20 marine macroalgae suggests that fluorescence emission could be used to distinguish macroalgal classes. Narrow waveband light was used to excite groups of accessory pigments in brown, red and green macroalgae and fluorescence emission measured at 685 nm, the wavelength at which chlorophyll a has its maximum fluorescence emission. Fluorescence induced by 540 and 465 nm wavelengths gave 540:465 fluorescence excitation ratios of 0·59±0·07 for seven brown algae, 3·67±0·56 for seven red algae and 0·28±0·07 for six green algae. These results suggest that fluorescence excitation signatures are relatively uniform within phylogenetic classes but differ substantially between classes. WhenEctocarpus siliculosus(Dillw) Lyng (Phaeophyceae) was cultured over a large range of light and nitrate regimes, the 540:465 fluorescence excitation ratio showed little variation. Somewhat higher fluorescence excitation ratios were correlated with tissue C:N ratios of 6·1–1·5. The ability to distinguish macroalgal divisions on the basis of fluorescence emission may allow the type and abundance of subtidal macroalgae to be characterized by existing laser-induced fluorescence methodology from low-flying aircraft.