The main objective of this work is to examine distribution of Lithuanian loaches, abundance and difference of their populations in Lithuanian inland waters, as well as factors conditioning selection of habitats, determine the taxonomic status and distinguish forms, subspecies or species. Novelty of the study: Loach has not yet been investigated in Lithuania, and earlier investigations into ichthyocenoses do not reflect the status, distribution, abundance and status of loaches. In dissertation are examined loach population status, distribution and abundance in Lithuania. The northern golden loach which is a new species for Lithuania was detected during this investigation and described and examined. The complex structure of spined loach populations is examined, which reveals the existence of now polyploidy hybrids in Lithuania. The knowledge of distribution of the golden loach is extended on a European scale. New information on biotypes of Cobitis taenia complex is presented and their biogeography is supplemented. The dissertation consists: The dissertation consists of the following chapters: Introduction, Literature Review, Material and Methods, Research Results and Discussion (consisting of 6 subchapters), Conclusions, Recommendations, References, List of Author’s Publications and Conference Abstracts. All the material is presented in 168 pages; Material and Methods, and Research Results and Discussion are presented in 103 pages. The list of references includes 137 sources. The text contains 74 figures and 21 tables.