Growth and chemotactic behavior in oxic^anoxic gradients were studied with two freshwater and four marine strains of sulfatereducing bacteria related to the genera Desulfovibrio, Desulfomicrobium or Desulfobulbus. Cells were grown in oxygen^sulfide countergradients within tubes filled with agar-solidified medium. The immobilized cells grew mainly in the anoxic zone, revealing a peak below the oxic^anoxic interface. All tested strains survived exposure to air for 8 h and all were capable of oxygen reduction with lactate. Most strains also oxidized sulfide with oxygen. Desulfovibrio desulfuricans responded chemotactically to lactate, nitrate, sulfate and thiosulfate, and even sulfide functioned as an attractant. In oxic^anoxic gradients the bacteria moved away from high oxygen concentrations and formed bands at the outer edge of the oxic zone at low oxygen concentration ( 6 5% O 2 saturation). They were able to actively change the extension and slope of the gradients by oxygen reduction with lactate or even sulfide as electron donor. Generally, the chemotactic behavior was in agreement with a defense strategy that re-establishes anoxic conditions, thus promoting anaerobic growth and, in a natural community, fermentative production of the preferred electron donors of the sulfate-reducing bacteria.