Eleven species or functional types of heterotrophic bacteria were isolated during the study of a heterotrophic succession of an aquatic microcosm. Bacterial populations changed rapidly in numbers and in dominant types. The dominant primary producer, Chlorella, was dependent upon thiamin supplied by 5 of the 11 bacterial isolates. Bacteria were functional in four ways: as contributors of thiamin, CO2and possibly sources of nitrogen to primary producers; as primary consumers of algal excretions; as secondary consumers of bacterial excretions and as decomposers. During succession net photosynthesis (Pn) surpassed night respiration (Rni) from day 5 through 45; as maturity approached the Pn/Rniratio fluctuated around 1. Dissolved carbon was present at 240 mg C/liter at zero time and decreased owing to the heterotrophic activity of 1.5 x 109bacterial cells/ml by day 8. Dissolved carbon was maintained at approximately 20.7 mg C/liter from day 8 through maturity. Particulate biomass increased steadily until day 50, after which there was no further increase. A bimodal pattern was apparent in bacterial numbers, Chlorella numbers, community metabolism, C14—U—glyoxylate uptake and bacterial species diversity in which peaks were recorded early and late in succession while a mid—succession decrease in activity occurred. The microcosm exhibited trends expected of larger, more open, developing ecosystems. The microcosm and methods used in the study are useful laboratory tools for examining various ecological principles.