The algae from the Prototheca genus are very interesting and unique microorganisms belonging to the plant kingdom (Regnum Vegetabile), which are capable of infecting animals and humans, causing a disease called protothecosis. For a long time, algae of the genus Prototheca were not recognized as pathogens dangerous to humans. Possible isolations of these microorganisms from wounds, blood or faeces were usually interpreted as saprophytes or impurities. The objective of the study is presentation of the current state of knowledge concerning the biology, identification and pathogenicity for animals and humans caused by achlorophyllic algae of the genus Prototheca. Brief description of the state of knowledge. Protothecosis is a fairly rare infection caused by the achlorophyllic algae of the genus Prototheca, which are widespread in the environment.These algae are pathogens for animals and humans. Prototheca spp. may cause human infections, but they are also a problem in animal infections, mainly when it comes to mastitis in dairy cows, causing far-reaching economic losses. The structure of Prototheca algae makes them resistant to most fungistatics, antibiotics, and disinfectants, and is also resistant to high temperatures. Summary. Due to the capability of infecting animal and human organisms, and wide resistance to many factors and agents, it is important to deepen the knowledge concerning Prototheca algae and better recognize these extremely interesting microorganisms, in order to be able to cure protothecosis more effectively in the future. Considering an increasing number of people with reduced immunity this disease may affect an increasing number of humans; however, also animals.