Hydrocarbon-using bacteria are a major portion of the aquatic microflora. They are supported in part with natural hydrocarbon sources such as terpenes along with other classes of organics. Concentrations are small, in the μg L−1 range and generally below detection. The organisms are sometimes mischaracterized as high-affinity organisms due to small Michaelis constants, and measured affinities for these and other substrates of oligobacteria are much lower than for most organisms in laboratory culture. “Specific affinity” theory was used to predict the identity and concentrations of some naturally occurring hydrocarbons. Hydrocarbon concentrations are limited by both solubility and microbial kinetics at the steady state in an equilibrium that is analogous to chemostat culture. Both dissolved and oil phases of hydrocarbons are used. Microbial oxidation of the latter can help to cause a heavy oil to sink. Accumulation is thought to be by partitioning into the membrane lipid portion of the cell envelope for partial oxidation and trapping into the interior. One consequence of this “vectorial partitioning” is the liberation of partly oxidized hydrocarbon degradation products into the environment.