EN
Mass occurrence of nuisance algal species Gonyostomum semen is observed in European humic lakes since 1970s, initially in the Scandinavian countries, then in eastern, central and western part of the continent. In 2002 mass appearances of this flagellate were found in three of 12 investigated humic lakes situated in the Eastern Poland. Lakes with Gonyostomum were situated within small geographical area. The biomass of algae was usually higher than 1 mg dm⁻³ and during summer its distribution was frequently uneven with higher values found in deeper layers. Based on this research we conclude, that G. semen during its spreading on the new area had preferred 6–8 m deep and small lakes with thermal and oxygen stratification as well as with low calcium content, slightly acidic to slightly alkaline pH and moderate color of water.