Fluorescent reporter systems have multiple applications in biotechnologies, owing to their inherent capacity to make invisible biological processes visible and quantifiable. Despite numerous efforts, fluorescence reporter systems have been unusable for decades in anaerobic bacteria, as for classical fluorescent proteins such as GFP, chromophore maturation is O2-dependant. With the recent development of ligand-based fluorescent systems such as HaloTag or FAST (Fluorescence Activating and Absorption Shifting tag), the fluorescent protein technology has finally been amenable to anaerobic microbes. Proof-of-concept was indeed shown by other laboratories for different mesophiles such as C. acetobutylicum, C. ljungdahlii or A. woodii. In this work, we pioneered the use of FAST in the naturally competent, thermophilic acetogen Thermoanaerobacter kivui. We tested different promoters as well as two different FAST variants to evaluate whether this reporter system could be used in vivo at elevated temperatures (i.e., 66°C). Preliminary results indicate that FAST can indeed be used to monitor genetic expression in T. kivui, demonstrating the applicability of FAST to thermophilic microorganisms.