The effect of white, red, yellow, green and blue light on the size of phyco- and photobiont cells of 9 lichen species from Knyszyńska Forest were investigated in laboratory conditions. Measurement of the phyco- and photobionts under the microscope shows three size groups of cells: young cells, fully grown vegetative cells and autosporangia. The light producing with highest percentage of young cells was considered to be the most favourable light for phyco- and photobiont cell propagation: white in the case of Hypogymnia physodes; white and red in Cladonia furcata; white and yellow in Peltigera canina; white, red and yellow in Pseudevernia furfuracea; red, blue and green in Xanthoria parietina; green in Cetraria islandica, Evernia prunastri, Platismatia glauca, and blue and green in Ramalina fraxinea.