To help understand the mechanism and control of Hg uptake in Hg-methylating bacteria, we investigated the effect of sulfide on Hg methylation by pure cultures of the sulfate-reducing bacterium Desulfobulbus propionicus (1pr3). Our previous research in natural sediments has suggested that Hg methylation occurs most rapidly when sulfide concentrations favor formation of neutral dissolved Hg-S species. In this study, the chemical speciation of Hg in culture media was manipulated by growing D. propionicus across a range of sulfide concentrations, with inorganic Hg (HgI) added in the form of ground ores. A solid-phase, rather than a dissolved source of Hg, was used to simulate the controls on Hg partitioning between solid and aqueous phases found in natural sediments. Methylmercury (MeHg) production by cultures was not related to the absolute solid-phase concentration of Hg in the ores, and it was only weakly related to the dissolved HgI concentration in the medium. However, MeHg production was linearly related to the calculated concentration of the dominant neutral complex in solution, HgS degrees. Furthermore, the diffusive membrane permeability of HgS degrees, as estimated from its octanol-water partitioning coefficient, was found to be sufficient to support MeHg production by cells. The present paper expands on our previous work by providing experimental support of our hypothesis that sulfide influences methylation by affecting the speciation of dissolved HgI and its uptake via passive diffusion.