The tolerance of halophilic archaebacteria towards bromide was tested in view of the fact that bromide occurs in natural brines in concentrations of up to 66 m M. It was found that, while concentrations of up to 0.8-1 M are tolerated well by all halobacterial types examined, great differences exist between species with respect to bromide tolerance. While Halobacterium (H. salinarium, H. halobium, and H. sodomense) and Natronobacterium species are only moderately tolerant, Haloarcula (H. vallismortis, "H. marismortui"), and Haloferax species (H. mediterranei, H. gibbonsii) tolerate higher concentrations. Haloferax volcanii proved extremely tolerant and showed growth in bromide media at very low chloride concentrations (below 50 m M). No correlation was found between bromide tolerance and the bromide concentration in the habitat from which the strains were isolated. Iodide proved much more toxic than bromide. Bromide-tolerant strains also proved relatively resistant to growth inhibition by iodide.