The in situ productivity of both the phytoplankton and the macro- phytes in Marion Lake, B.C., was determined from April 1966 through September 1966, by using ¹⁴C techniques. The uptake of NaH¹⁴CO₃ was measured in selected macrophytes by enclosing them in plexiglass chambers. These plants were then combusted in oxygen, and the ¹⁴CO₂ was absorbed in toluene-POPOP-ethanolamine. Radioassay was accomplished by liquid scintillation. In addition to the ¹⁴C method, an organic weight method was used to measure macrophytic productivity. The productivity of the macrophytes was always higher than that of the phytoplankton. There was a considerable difference in the estimates of the macrophyte productivity arrived at by the two different methods, and reasons for this are discussed. The total productivity of the lake is very low when compared with lakes of similar latitudes because of low phytoplankton productivity. It is concluded that in Marion Lake the macrophytes are more important primary producers than the phytoplankton. The difficulty of comparing data between this and other studies is discussed, and the need for standardization of methods is emphasized. / Science, Faculty of / Zoology, Department of / Graduate