Ultrastructure and functional characteristics of photosynthetic apparatus of the floating and submersed leaves of heterophyllous aquatic plant Nuphar lutea (L.) Sibth. et Sm. were examined. Differences in the mesophyll cell chloroplast ultrastructure, pigment content and chlorophyll fluorescence parameters of floating and submersed leaves as well as of submersed leaves at the different water depth were detected. The efficiency of PS II was established to reduce significantly under the chlorophyll fluorescence induction with light intensity higher than 60 micromol m(-2)s(-1). The obtained results may be considered as plant adaptation to reduced light intensity and changed light spectrum in the water.