The tropical marine sponge, Halichondria phakellioides, from Darwin Harbour contains high concentrations of molybdenum. A rod-like bacterium extracellular in sponge tissue was observed using transmission electron microscopy. Molybdenum was located within these bacteria, but not in sponge cells. This is the first report of the trace element molybdenum localised in a sponge bacterial symbiont. Many different bacterial symbionts were identified in the sponge by sequence analysis so the identity of the molybdenum-accumulating bacterium could only be inferred.