Between the years 1965 and 1967, phytoplankton collections were taken in the summer from nearly 200 lakes in a diversity of areas throughout the State. About 220 taxa of euplanktonic algae were identified. The analysis of the Minnesota lake phytoplankton indicates there is a diminution in species diversity in the course of the evolutionary progression as lakes change in character from oligotrophy to eutrophy. Many of the markedly eutrophic lakes have suffered severe disturbance of the natural system due to artificial enrichment. Eutrophic lakes in Minnesota are typically dominated in summer and early fall by water blooms of blue green algae as is usual in most productive lakes of temperate regions. Microcystis aeruginosa, M. wesenbergii, Coelosphaerium naegelianium, Alphanizomenon flos aquae, numerous species of Anabaena, Lyngbya birgei, and Gloeotrichia echinulata are most common.