The interactions between the component species of a mixed bacterial culture grown on methane were investigated. The culture contained the methane-utilizing bacterium Methylo- coccus sp. and four heterotrophic bacteria which were unable to grow on methane but utilized products released by lysis of the Methylococcus sp. Products of methane oxidation such as methanol, formaldehyde and formate did not constitute a significant amount of the organic carbon found in the culture supernatant of the pure methane-utilizing bacterium grown in chemostat culture. The organic carbon was accounted for mainly as protein and nucleic acid and originated from a growth-dependent lysis of the methane-utilizing bact- erium. In the mixed culture the level of the organic carbon was reduced and it is sug- gested that extracellular enzymes produced by the heterotrophic bacteria play a role in the degradation and utilization of these compounds. The major heterotrophic bacterium Pseudomonas sp. NCIBI 13 LO produced an extracellular neutral protease that degraded the proteins present in the culture supernatant of the methane-utilizing bacterium.