A series of radiative transfer calculations is performed to study the potential of using Sun-stimulated chlorophyll fluorescence for the detection of phytoplankton. Calculated radiance spectra including the fluorescence peak agree well with measurements. We found that the Sun-stimulated natural fluorescence of chlorophyll-a is a good predictor for phytoplankton, even in waters with varying suspended matter and yellow substance concentrations. An increase in chlorophyll concentration of 1 mg m−3causes an increase in fluorescence of about 0·05Wm−2sr−1μm−1, when a fluorescence efficiency factor of 0·3 per cent was assumed. The vertical stratification of chlorophyll affects the fluorescence signal, for example, the fluorescence is reduced to 30 per cent, if the phytoplankton is below 2 m depth instead of reaching the water surface.