A study was made of diurnal changes in the heterotrophic uptake kinetics of both ¹⁴C-glucose and ¹⁴C- glycolate in two small stratified lakes, Lake Rotomanuka in New Zealand and Lake Fryxell in Antarctica. The purpose of the investigation was to study the effect that diurnal changes in the extracellular release of certain substrates by algae might have on the uptake of those substrates by aquatic microheterotrophs. The heterotrophic uptake technique of Parsons and Strickland.(196l) was modified to provide a technique which was more convenient for field-work. In an effort to exclude the effects of migration and heterogeneous population distribution on substrate uptake, the microbial population in each lake was observed using a combination of (a) direct counts of bacteria (using acridine orange staining and epi-fluorescent microscopy); (b) measurement of chlorophyll α and bacterio-chlorophyll and (c) measurement of the reduction of INT dye. The natural concentration of glycolate was measured on some occasions using a modification of the technique of Shah and Wright (1974).