Given the role of algae as primary producers and the possibility of reduction in their numbers by some fish species, as well as possible food competition between some autochthonous and allo‐chthonous fishes which are consumers of algae, feeding of two autochthonous species (carp and roach) and two allochthonous species (crucian carp and bighead carp) was investigated. Investigations were carried out in the following aquatic ecosystems: Sta‐ri Begej, Palicko Jezero and Bukinski Rit which are parts of the legally protected areas (nature reserves). All investigated aquatic ecosystems are eutrophic ones, characterized by high density of phytoplankton communities in which Bacillariohyta and Chlorophyta are the dominant algal groups. Feeding spectra preliminary investigation showed that algae take an important place in feeding of investigated fish species. Analyses of the algal component showed that participation of the different algal groups varied, but, generally, Bacillariophyta and Chlorophyta were dominant, which correspond to their domination in the waters. Detailed analyses of the algal component showed that, concerning the contribution of algal groups, there were no significant differences between the investigated fish species. It suggests likely possibility for food competition, as well as that some autochthonous species, such as the carp and roach, can decrease the density of algae as successful as the allochthonous ones, such as the crucian carp and bighead carp. Given that all of the investigated water bodies are the parts of protected areas characterized by a high diversity of autochthonous species, and that the allochthonous species can be a real threat to them, it is necessary to carry out measures directed to reducing the numbers or eliminating the allochthonous species from the protected water bodies.