Aero-terestric algae occure on a wide range of substrates, such as soil, plant, stone and rock surfaces and also on a variety of anthropogenic substrates. Morphological convergence towards coccoid or filamentous thalli, caused by frequent dessication stress, is typical for this group. Algae are frequently used for biomonitoring of various biotopes, mostly water biotopes, but also for bioindication of air quality. My diploma thesis is based on a study of autotrophic microbial growths on spruce and yew needles in relation to various air quality measures evaluating air pollution of sites. The results show that the occurence of microbial growths on needles is primarily influenced by the orientation towards cardinal points. However, the air pollution factors also proved to be significantly related to the quantitative measures of the microbial assemblages. Among them, the relative proportion of algae in microbial growths was the most tightly related to air pollution.