The inhibition of dehydrogenase activity in Lysinibacillus fusiformis by quaternary, quinary and senary mixtures of Sodium Dodecyl Sulfate (SDS) and some heavy metals was assessed as a mark of their toxicities. The bacterium was isolated from Talinum fruticosum farms irrigated with Otamiri river water, in Owerri, Imo State, Nigeria. Fixed ratio mixtures (Arbitrary concentration ratio (ABCR) and equi-effect concentration ratio (EECR) mixtures) were used to assess the joint toxicities of the toxicants. Also, concentration addition (CA) and independent action (IA) models were used in predicting the effects of the toxicants on the soil bacterium. Based on their EC50S, Cd was the most toxic among the individual toxicants (0.013± 0.001 mM), followed by Co (0.035±0.003 mM), and SDS the least (2.613±0.173 mM). Duncan tests indicated that the EC50S of the toxicants statistically differed from one another (P<0.05), with the toxicity ranking as Cd > Co > Zn > Ni > Pb > SDS. The responses of the bacterium to the toxicants effects were dose-dependent, and the toxicants also progressively inhibited the dehydrogenase activity as the concentration increased. All the dose-response relationships of the ABCR and EECR mixtures and the individual components could be described by 2-parameter logistic function. Most of the mixtures were strongly synergistic against the bacterium. Within each mixture ratio, experimentally-derived, CA- and IA predicted EC50S were statistically different from one another in all the tested mixtures (P<0.05). The quinary mixtures were the most toxic among the studied mixtures. Both models greatly underestimated the toxicities of the mixtures against L. fusiformis. The synergistic effects of the mixtures indicate their potential harmful effects on the soil bacteria.