1. Twelve bacterial isolates that grow with trimethylamine as sole source of carbon and energy were obtained in pure culture. All the isolates grow on methylamine, dimethylamine and trimethylamine. One isolate, bacterium 4B6, grows only on these methylamines whereas another isolate, bacterium C2A1, also grows on methanol but neither grows on methane; these two organisms are obligate methylotrophs. The other ten isolates grow on a variety of C(i) and other organic compounds and are therefore facultative methylotrophs. 2. Washed suspensions of the obligate methylotrophs bacteria 4B6 and C2A1, and of the facultative methylotrophs bacterium 5B1 and Pseudomonas 3A2, all grown on trimethylamine, oxidize trimethylamine, dimethylamine, formaldehyde and formate; only bacterium 5B1 and Ps. 3A2 oxidize trimethylamine N-oxide; only bacterium 4B6 does not oxidize methylamine. 3. Cell-free extracts of trimethylamine-grown bacteria 4B6 and C2A1 contain a trimethylamine dehydrogenase that requires phenazine methosulphate as primary hydrogen acceptor, and evidence is presented that this enzyme is important for the growth of bacterium 4B6 on trimethylamine. 4. Cell-free extracts of eight facultative methylotrophs, including bacterium 5B1 and Ps. 3A2, do not contain trimethylamine dehydrogenase but contain instead a trimethylamine monooxygenase and trimethylamine N-oxide demethylase. It is concluded that two different pathways for the oxidation of trimethylamine occur amongst the isolates.