Lake Osbysjon, near Stockholm, is small (46,000 m2) and shallow (3.5 m). The results of the analysis of 180 phytoplankton samples taken in 1957 and 1958 are reported. Determinations were made where possible on living samples, but samples were preserved in HCHO or I2-KI-acetic acid also. Estimates of the volumes of phytoplankters in [mu]3/i are given. A list is given of the phytoplankters observed, with a brief description of their seasonal and vertical distributions. The chrysophytes dominated with the exception of the winter months, when bacteriophytes and dinoflagellates were of greater importance. At no time were the diatoms of any great significance. Calculations were made of the total biomass of the phytoplankton in the lake (assuming the density of the phytoplankton species approximated one.) Values ranged from 47.4 kg in September 1957 to 360 kg in late April of 1958. At the end of July there was marked vertical stratification of the phytoplankton. Similar stratification occurred in December and January, when most of the bacteriophytes and small monads were near the bottom. The winter phytoplankton was dominated by nannoplankters.