Growth of two dissimilatory sulfate-reducing bacteria, Desulfosporosinus orientis (gram-positive) and Desulfovibrio desulfuricans (gram-negative), in a chemically defined culture medium resulted in similar growth rates (doubling times for each culture = 2.8 h) and comparable rates of H₂S generation (D. orientis = 0.19 nmol/L S^2–per cell per h; D. desulfuricans = 0.12 nmol/L S^2–per cell per h). Transmission electron microscopy of whole mounts and thin sections revealed that the iron sulfide mineral precipitates produced by the two cultures were morphologically different. The D. orientis culture flocculated, with the minerals occurring as subhedral plate-like precipitates, which nucleated on the cell wall during exponential growth producing extensive mineral aggregates following cell autolysis and endospore release. In contrast, the D. desulfuricans culture produced fine-grained colloidal or platy iron sulfide precipitates primarily within the bulk solution. Mineral analysis by scanning electron microscopy – energy dispersive spectroscopy indicated that neither culture promoted advanced mineral development beyond a 1:1 Fe:S stoichiometry. This analysis did not detect pyrite (FeS₂). The average Fe:S ratios were 1 : 1.09 ± 0.03 at 24 h and 1 : 1.08 ± 0.03 at 72 h for D. orientis and 1 : 1.05 ± 0.02 at 24 h and 1 : 1.09 ± 0.07 at 72 h for D. desulfuricans. The formation of “biogenic” iron sulfides by dissimilatory sulfate-reducing bacteria is influenced by bacterial cell surface structure, chemistry, and growth strategy, i.e., mineral aggregation occurred with cell autolysis of the gram-positive bacterium.