A highly motile chemoautotrophic strain of hydrogen sulfide-oxidizing bacteria from coastal seawater produces solid sulfur filaments of dimensions 0.5 to 2.0 micrometers by 20 to 500 micrometers. Filamentous sulfur is rapidly produced by direct excretion by a vibrioid organism, and the newly produced filaments are thickened by the deposition of sulfur by other members of the population. Microscopic observations of the flocculent discharge material collected from diffuse-flow hydrothermal vents (9 degrees N, East Pacific Rise) revealed that the material from this source is composed largely of filamentous sulfur of morphology nearly identical to that obtained in the model laboratory system.