The archaebacterium Methanococcus voltae, was shown to be chemotactic. Acetate, isoleucine, and leucine were identified as attractants; whereas histidine was not an attractant. A motile, generally nonchemotactic mutant was isolated.Tactic responses have been observed in a wide range of organisms in all three of the urkingdoms: the eucaryotes (4, 13), eubacteria (2,6,7,12), and archaebacteria (10). The archaebacteria fall into two major divisions (15). Division I comprises the sulfur-dependent thermophilic archaebacteria. Division II consists of the three major methanogen families, extreme halophiles, and two types of thermoacidophiles. The family Methanococcales is separated from the other methanogens and relatives by the deepest division in that grouping. It was therefore of interest whether Methanococcus voltae, a member of the Methanococcales and thus distantly related to the chemotactic halophile Halobacterium halobium, shows chemotactic behavior. Since M. voltae is motile and transports amino acids (5), it seemed plausible that M. voltae would be able to respond to its environment. Here we report evidence of chemotaxis in M. voltae and the isolation of a mutant defective in general chemotaxis.In addition to H2 and CO2, M. voltae requires acetate, isoleucine, and leucine for growth (11,14). Although it has been shown that neither metabolism nor transport is required for sensing an attractant in Escherichia coli (8), these three compounds were reasonable substances to test as attractants. To determine if M. voltae was chemotactic toward any of these compounds, cells were spotted on acetate, leucine, or isoleucine swarm agar gradient plates. Gradient swarm agar plates were made by pouring 10 ml of swarm agar (0.35%) in defined medium (14) containing 10 mM sodium acetate, 7.5 mM L-isoleucine, and 7.5 mM L-leucine into a petri dish and allowing the agar to solidify on an angle. Ten ml of defined swarm agar lacking acetate, isoleucine, or leucine was then added to leveled plates. Approximately 107 washed cells in 10 ,ul of 0.4 M NaCl were spotted on the side of the plate with the lowest concentration of solute. Unless otherwise stated, all plating and assay manipulations were performed in an anaerobic glove box (Coy Laboratory Products, Ann Arbor, Mich.) with anaerobic solutions. The plates were incubated in brass and stainless steel cannisters under a H2-CO2 (80:20) atmosphere to which H2S was added to a final concentration of 0.2%. Over a 1-week period, M. voltae moved toward the higher concentrations of acetate and leucine but not isoleucine. However, isoleucine was later identified as an attractant with a higher threshold concentration in the capillary tube assays. M. voltae may not have responded to the isoleucine gradient in swarm agar plates because the cells were not placed on the plate at the threshold concentration.Chemotaxis assays were performed by the method of * Corresponding author.Adler (1)