The lucinid bivalve Codakia orbiculata, whose gills contain sulphur-oxidizing symbiotic bacteria, occurs in high densities (500/m2) in the rhizosphere of shallow-water Thalassia testudinum sediments in Bermuda. Both sulphide and thiosulphate stimulate aerobic respiration in the isolated bacterial symbionts of C. orbiculata. Sulphide and thiosulphate stimulate anaerobic ^^COa fixation in bacteria isolated from sulphur-starved bivalves. Interstitial water sulphide concentrations in the bivalves' habitat reach 300 pM, and sulphate-reduction rates are high, but thiosulphate concentrations are low (0.66-32.27 pM) . Thiosulphate supplied to the symbionts in vivo must be produced by sulphide oxidation, possibly by the host bivalve. Isolated symbionts also respire aerobically and fix i * C02 in the absence of exogenous reduced sulphur, suggesting utilization of intracellular elemental sulphur stores. Codakia orbiculata symbiotic bacteria are able to respire nitrate. Nitrate concentrations in the interstitial water of C. orbiculata habitat can reach 36 pM. Thiosulphate stimulates nitrate respiration in the intact symbiosis, incubated in oxic and anoxic conditions, and in anoxic incubations of isolated symbionts. Intracellular elemental sulphur is also used by the •bacteria as a substrate in nitrate respiration. Nitrate respiration in the absence of exogenous nitrate suggests that the sjnnbionts may have a limited ability to store nitrate. There is no direct evidence that sulphide stimulates nitrate respiration in either the isolated symbionts or the intact symbiosis, incubated in anoxic conditions. Nitrite respiration in the symbionts is stimulated by sulphide (only), however. Because nitrate respiration was measured by nitrite accumulation, complete denitrification would explain the apparent failure of sulphide to stimulate nitrate respiration. High nitrate respiration rates in the intact symbiosis, incubated with sulphide in oxic conditions, may be in response to thiosulphate, supplied to the bacterial symbionts after host oxidation of sulphide. Nitrite respiration in the intact symbiosis, even when incubated in oxic conditions, demonstrates that the symbionts have access to some sulphide in vivo, however, and that host sulphide-oxidation may not be- 100% efficient. Nitrate and nitrite respiration in the intact symbiosis, even when incubated in oxic conditions, suggests that the bacteria may be exposed to low oxygen levels in vivo and may require the ability to utilize an alternate electron acceptor. Like some free - living bacteria , Codakia orbiculata bacterial symbionts may co-respire, or alternately respire , oxygen and nitrate . The Thalassia testudinum sediments in Bermuda may be ideal for this bacteria-bivalve symbiosis due to the availability of oxygen, nitrate and sulphide.