Hydrotesting is commonly used in industrial systems. Incomplete or improper hydrotesting can cause microbiologically influenced corrosion (MIC). Microorganisms present in water, such as sulfate-reducing bacteria and iron bacteria, can either initiate MIC in protected systems or enhance it in underprotected installations. This article explains probable scenarios describing MIC in the case of inadequate hydrotesting. It suggests methods to reduce the risk of MIC during hydrotesting.