halobacterium salinarium (formerly H. halobium) is a chemotactic and phototactic archaeon from which volatile methyl groups are released continually, a phenomenon related to its sensory system. We found that released methyl groups comprised two different chemical species, methanol and methanethiol, the sulfur analog of methanol. Radiolabeling experiments showed that the methyl groups of both compounds, as well as the sulfur of methanethiol, were derived from methionine but were donated to cellular components and subsequently cleaved to produce the respective volatile compounds. Previous work had shown that chemostimuli and photostimuli result in transient increases in the rate of release of volatile methyl groups. We found that these increases reflected increased release of methanol but not of methanethiol. Thus, the methyl group chemistry of the H. salinarium sensory system is analogous to the well-studied chemotactic system of Escherichia coli. The reactions that result in methanethiol release are of unknown function and have unusual features. They may involve a methionine-gamma-lyase activity we detected in H. salinarium. Sulfur derived from methionine was found attached to specific proteins in reduction-sensitive disulfide linkages.