Methanococcus jannaschii is an autotrophic archaeon originally isolated from an oceanic thermal vent. The primary metabolic pathway for energy production in this hyperthermophilic microbe is methanogenesis from H2 and CO2. As an autotroph, M. jannaschii requires only CO2 as a carbon source for synthesizing all necessary biomolecules. Changes in the environmental availability of these molecules can be expected to activate regulatory mechanisms manifested as the up and down regulation of specific genes and the concomitant increase and decrease in abundance of the corresponding proteins. In our analysis of the proteome of M. jannaschii, we have observed significant changes in the abundance of a common subset of predominant proteins in response to reduced H2 concentration, limited ammonium availability, and the stage of cell growth (exponential compared with stationary). The masses of tryptic peptides from these proteins match those predicted by M. jannaschii genome open reading frames annotated as flagellin B1 (MJ0891) and flagellin B2 (MJ0892). Multiple proteins with different isoelectric points and molecular weights match each of these proteins, and the abundance of these protein variants changes with growth conditions. These data indicate that structural modifications altering both the isoelectric point and size of the M. jannaschii flagellin B1 and B2 proteins occur in response to growth conditions and growth stage of M. jannaschii and further suggest the regulation of M. jannaschii motility through structural modifications of the building blocks of the flagella.