The effect of increasing concentration (0.01, 1 and 5 mg l(-1)) of fluoranthene (FLT) and the duration of exposure (12, 24, 36, 48 and 60 h) on the chlorophyll fluorescence parameters (F0, F(V)/F(M), Phi II and (F(M) - F(S))/F (S)) of symbiotic algae in thalli of two foliose lichens Lasallia pustulata and Umbilicaria hirsuta was investigated. Also the FLT concentration in thalli of both lichen species and the proportion of symbiotic algae (photobionts) in thalli was studied. The results obtained demonstrated that the applied concentration of FLT (1 and 5 mg l(-1)) affected primary photochemical processes of photosynthesis of algae in both lichen species. The F0 values increased and the F(V)/F(M) and Phi II values decreased. The fluoranthene content in thallus of both lichen species was increased with increasing FLT concentration in the environment. Chlorophyll fluorescence parameters showed different sensitivity to the content of FLT in Lasallia pustulata and Umbilicaria hirsuta, respectively. Higher sensitivity, detected in U. hirsuta, can be related to its diverse anatomical structure (higher portion of symbiotic algae in thalli) and to the different (nearly 2.5 times higher) content of FLT after exposure.