Seawater transparency estimated by means of the Secchi disc depth depends on all optically active water constituents, among them the chlorophyll a concentration. This parameter is very important for primary production studies, and it might be very useful to investigate its relationship to the water transparency measured in such an easy way. Long-term observations of the chlorophyll a concentration and the associated Secchi disc depth from three stations in the southern Baltic are presented. The data were collected during almost twenty five years (1971-1995). The correlation between these parameters and the influence of chlorophyll a concentrations on the Secchi disc depth are examined.