The occurrence of several heterotrophic parameters are described with reference to the oligotrophic waters of West Blue Lake, Manitoba. The relationship between uptake velocities and concentration of nine organic acid substrates (lactic, pyruvic, fumaric, malic, acetic, succinic, glycolic, citric and formic acids) upon kinetic analysis produced values (Vmax, T, K + S and v values) which in most cases were of the same order of magnitude as reported in the literature for bacterial populations. Except for one substrate (formic acid) uptake velocities increased in the light and the dark with time, in most cases with lag in uptake velocity being present during 48 hr experiments. Such lags were felt to indicate periods of increase in bacterial populations within sample bottles. The relationship between uptake velocity and depth for three of the nine substrates (lactic, malic and succinic acids) produced responses which were believed to be associated with the temperature profile and nutrient availability of the water column. Evidence is given for the presence of different bacteria utilizing lactic and succinic acid than malic acid. Relative heterotrophic productivity as estimated for these three substrates was believed to be greater than primary productivity on a day to day basis. "In situ" experiments using NaH 14CO3 have shown that initial uptake rates were highest for 12 to 24 hr experiments and that subsequent decreases in uptake rates with time indicated re-assimilation of excreted products. It was felt that an equilibrium response between the organisms and the confines of the samples bottle in addition to a coupled oscillatory response were in operation controlling excretion and re-assimilation. Loss of activity during filtration of samples was investigated but no such response was measured.