We measured sulfate reduction in the acidic (pH less than or equal to 3) sediment of an Argentinean lake influenced by volcanism. Sulfate reduction rates of 2.04 mmol m(-2) d-(1) were determined with a (SO42-)-S-35 core injection method and confirmed by batch incubations and from H2S measurements in the sediment. H2S production stopped when iron reduction was stimulated by addition of ferric iron. The results suggest that sulfate reduction at pH values around 3 is possible and can probably be used in biotechnological strategies if competing microbial processes are inhibited and electron donors are highly available.