Transport of the osmoprotectant glycine betaine was investigated using the glycine betaine-synthesizing microbe Methanohalophilus portucalensis (strain FDF1), since solute uptake for this class of obligate halophilic methanogenic Archaea has not been examined. Betaine uptake followed a Michaelis-Menten relationship, with an observed K_t of 23 µM and a V_max of 8 nmol per min per mg of protein. The transport system was highly specific for betaine: choline, proline, and dimethylglycine did not significantly compete for [¹⁴C]betaine uptake. The proton-conducting uncoupler 2,4-dinitrophenol and the ATPase inhibitor N,N-dicyclohexylcarbodiimide both inhibited glycine betaine uptake. Growth of cells in the presence of 500 µM betaine resulted in faster cell growth due to the suppression of the de novo synthesis of the other compatible solutes, -glutamate, -glutamine, and N-acetyl--lysine. These investigations demonstrate that this model halophilic methanogen, M. portucalensis strain FDF1, possesses a high-affinity and highly specific betaine transport system that allows it to accumulate this osmoprotectant from the environment in lieu of synthesizing this or other osmoprotectants under high-salt growth conditions.