Production of bacterioplankton was studied together with phytoplankton composition at a pelagic site in the north basin of Lake Biwa from 19 February to 7 June 1990. During the investigation period, two species of phytoplankton predominated: Asterionella formosa (Bacillariophyceae) from 19 February to 18 April (the early period) and Uroglena americana (Chrysophyceae) from 9 May to 7 June (the late period). Bacterioplankton production estimated by the frequency of dividing cells method remained at a high level during the early period, ranging from 7.4 to 33μgC⋅l^-1⋅d^-1. During the late period, bacterioplankton production was low, ranging from 4.1 to 18μgC⋅l^-1⋅d^-1. The change in bacterioplankton production appeared to relate to the change in phytoplankton composition.