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Abstract: Microbiologically influenced corrosion (MIC) is caused by microbial biofilms. In this work, an oilfield produced water sample was analyzed using a newly developed disposable electrochemical biofilm/MIC test kit consisting of two solid-state electrodes in a 10 mL standard serum vial for assessing biofilm growth, biocorrosivity and biocide treatment efficacy. The produced water sample was found to be low in microbial cell counts and nutrients. To simulate a possible worst-case scenario, the produced water sample was subcultured at 37 °C using enriched artificial seawater (EASW) for 3 rounds before being used as the seed culture for further MIC and biocide tests. The electrochemical test results from the 10 mL biofilm/MIC test kit including polarization resistance (Rp) from linear polarization resistance scans and corrosion current density (icorr) from Tafel scans indicated a corrosion rate sequence of no biocide treatment >20 ppm (w/w) tetrakis hydroxymethyl phosphonium sulfate (THPS) > 50 ppm THPS. Rp was able to predict biofilm maturity time using the incubation time when Rp leveled off (i.e., time to reach maximum corrosivity). Two common electron transfer promotors were found to accelerate MIC in the test kit vial injection tests, pointing to extracellular electron transfer-MIC as the main mechanism. This observation was consistent with the 30% corrosive sulfate reducers among all microbes in the mixed culture sample found by metagenomics. In the coupon incubation tests in 125 mL anaerobic vials, the 7-d X60 carbon steel weight loss was 1.1 ± 0.2 mg/cm2 (2.9 mpy uniform corrosion rate) without biocide treatment. With 20 ppm THPS biocide in EASW, it dropped to 0.5 ± 0.2 mg/cm2 (1.3 mpy), and with 50 ppm THPS, it became negligible. The corresponding MIC pit depths were 10.5, 8.9 μm, and no well-defined pits, respectively for the three biocide treatment conditions. The weight loss data confirmed the corrosion rate sequence from the biofilm/MIC test kit. This work presents a new MIC monitoring and biocide treatment assessment system for oilfield applications using the new biofilm/MIC test kit.