Methyl‐accepting taxis proteins were identified and characterized in Halobacterium halobium, an archaebacterial species that is both chemotactic and phototactic. The data suggest direct involvement of methylation and demethylation in mechanisms of both chemotaxis and phototaxis and identify adaptation as the sensory process in which those reactions are likely to be involved. Analysis by electrophoresis and fluorography revealed methyl‐accepting species, of apparent Mr between 90,000 and 135,000, that exhibited characteristics of sensory components. Those methyl‐3H‐labeled species were absent in a mutant blocked in taxis. Methylation of specific bands increased after positive chemostimuli and decreased after negative stimuli. Other methyl‐3H‐labeled bands, from 17 to 29 kd, exhibited features of biosynthetic intermediates, not of sensory components. Assay of rates of demethylation by measuring release of volatile forms of radiolabeled methyl groups revealed transient changes following chemo‐ or photostimuli that persisted for periods roughly equivalent to adaptation times. Negative chemostimuli induced increased rates of demethylation, as expected from fluorographic analysis, but positive chemostimuli also resulted in an increase. Photostimuli of either sign were followed by increases in rates of demethylation of shorter duration and lesser magnitude than chemostimuli‐induced increases, a relationship that corresponded to differences in adaptation time.