A new biosensor, the RANTOX (Rapid ANaerobic load and TOXicity tester), primarily designed for anaerobic process control, has been tested to evaluate its potential to detect toxic loads in wastewaters. The biosensor is made of a laboratory-scale reactor. Two different prototypes have been tested: an expanded granular bed anaerobic reactor (EGBR), fed on synthetic solutions, and a fluidized anaerobic reactor fed on yeast production wastewaters. Sodium chloride was used as a toxicant because it reversibly inhibits acetoclastic methanogens and allows the carrying out of repeated experiments on the same biomass. According to the experimental results, the RANTOX biosensor could be successfully used to monitor full-scale anaerobic reactors and detect unexpected toxicants in their feed.