An ecological and floristic phytoplankton assessment of the Ob River in one of its middle course segments, located at high latitudes, was carried out. A rich algological community was observed, based on the summer and autumn field material, with the composition noticeably changed since earlier studies carried outin 1996. The top 10 families comprise more than a half, and the largest 8 genera include 33% of all observed algae.Under conditions of pH close to neutral, the main factor influencing the formation of species diversity, total abundance and structure of the identified phytoplankton is water temperature. Thestudy also features the floristic comparison of the Ob River plankton algocenoseswith those of other large rivers: Lena, Irtysh, Dnieper, Volga. The prevalence of planktonic, widespread, oligohalobial, betamesosaprobic varieties and forms of algae, which prefer a neutral medium reaction, was revealed in the studied phytoplankton.