The effects of a mixed culture of <i>Desulfovibrio</i> species and of a pure culture of <i>Thiobacillus ferrooxidans</i> on the corrosion resistance of electroless nickel-plated mild steel (ENP) were examined. A novel device for the qualitative microscopical investigation of microbiologically induced corrosion was developed for use with laboratory immersion tests. Quantitative weight loss determinations were also made on the metals. Sparse biofilm formation in the case of <i>Desulfovibrio</i> species resulted in low weight losses. <i>Thiobacillus</i> cultures rapidly colonized ENP surfaces, causing disintegration of the plating and pitting of the mild steel. These effects resulted in correspondingly high weight losses. The device used here provided a simple, rapid and convenient method of assessing corrosion resistance of metal formulations under aggressive laboratory conditions.