Microbiologically Influenced Corrosion (MIC) has been considered a significant factor contributing to oil and gas pipeline failures. This type of corrosion results from the activities of microorganisms in the biofilms formed on metal surfaces. Sulfate-reducing bacteria (SRB) are a predominant cause of MIC. The monitoring of MIC requires a combination of microbiological surface analytical and electrochemical methods and thus in-line MIC monitoring presents difficulties. In this paper an amperometric biosensor is constructed and tested for in-line detection of sulfide as a product of SRB activities. Carbon nanotubes functionalized with a polythiophene derivative conducting polymer were used to immobilize enzyme and facilitate signal transduction. The biosensor responded to the injection of sulfide linearly within 5 s. The sensitivity of the biosensor is improved by at least 3~5 folds compared to those using graphite powders for enzyme immobilization. The results paved the way for construction of in-line biosensors for fast and reliable monitoring of MIC.