The possibility of using natural fluorescence -- sun-induced chlorophyll a fluorescence (SICF) in remote sensing and/or contact investigations of chlorophyll a concentration (Ca) has often been discussed in recent years. The relationship between chlorophyll a concentration (Ca) and chlorophyll a fluorescence at 683 nm seems to evident and suggests a method of obtaining the Ca concentration on the basis of SICF intensity. Unfortunately this relationship is not as direct as we would like because SICF depends on numerous physiological properties of phytoplankton like fluorescence quantum yield and light absorption capacity which depend on an environmental factors. The influence of these parameters on SICF is shown in this paper and the relationships are presented. Using these we made attempt to achieve the direct functional dependence between the natural fluorescence (obtained from the upward reflectance coefficient spectrum measured at 665, 683 and 710 nm) and the chlorophyll a concentration. The analysis carried out in this study shows that the possibility determining Ca concentrations is limited to great optical depths in the sea ((tau) greater than 3). However using SICF for estimation Ca in the surface layer is probably impossible.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.