Desulfovibrio desulfuricans (API strain) was grown in an atmosphere of hydrogen on the agar surface of complex media, including yeast extract (Difco) and trypticase soy agar (BBL). Sodium lactate was required in the medium for the organism to grow on a 2% yeast extract agar surface in the absence of hydrogen (nitrogen atmosphere). The organism grew readily on the surface of trypticase soy agar (TSA) under nitrogen in the absence of an added hydrogen donor. A medium (TSA plus salts) is described based upon the addition of sodium lactate (4 ml/liter), magnesium sulfate (2 g/liter), and ferrous ammonium sulfate (0.5 g/liter) to TSA that appears suitable for the isolation and counting of Desulfovibrio on the surface of agar plates in an atmosphere of hydrogen. Sodium lactate does not appear to be essential in this medium for good growth and sulfate reduction in a hydrogen atmosphere, but is essential in a nitrogen atmosphere. In a hydrogen atmosphere growth of Desulfovibrio on the agar surface of media commonly used for its cultivation as well as on a completely inorganic medium containing bicarbonate as a source of carbon is poor and erratic unless inoculated (Desulfovibrio) plates of TSA plus salts are incubated in the same container with plates of these media. This stimulatory effect of incubation with inoculated plates of TSA plus salts medium appears to be due to an unidentified volatile material produced by Desulfovibrio desulfuricans when growing on this medium. Another volatile material or possibly the identical material appears to act as a hydrogen donor. (Author).