Inasmuch as phytoplankton is an important link in the food cycle of aquatic animals in a body of water, knowledge of its abundance might well indicate the productive capacity of that body of water. In the past, the majority of investigations involving quantitative phytoplankton collections and analyses have been limited to summer months. ... Because there may be one, two, or three phytoplnakton pulses in a lake annually, and since these may occur at any time of the year, it seems almost imperative to conduct year-round studies in order to gain a perspective of phytoplankton production. Plankton samples and physico-chemical data were collected from Douglas Lake for 16 consecutive months from July, 1950, through October, 1951. It is the main purpose of this paper to show the seasonal and vertical distribution of phytoplankton in Douglas Lake during that period of time and to discuss the possible roles of some of the physico-chemical conditions - particularly phosphorus - in the metabolism of a lake. In addition, plankton samples and physico-chemical data were collected during the summers of 1948, 1950 and 1951 from Munro, Vincent and Lancaster Lakes in northern Michigan and during the fall of 1950 from Lake Lansing in southern Michigan. These lakes are quite different morphometrically, and part of this paper will be devoted to a comparison of the phytoplankton and physico-chemical conditions of those lakes during the periods mentioned.