The paper presents the results of Secchi depth values recorded during the last 25 years in neritic as well as one pelagic station in the middle Adriatic. Trend of changes in Secchi depth indicated a negative sign at all investigated areas, that means transparency decreased from 0.02 to 0.08 in yr(-1) during that period. The effect of phyloplankton on the attenuation of light in the water also can be described in terms of the chlorophyll concentration.