Compounds of mercury have long been known to be extremely hazardous, but it was not until the poisonings at Minamata (1953) and Nigata, Japan (1965) that world-wide attention was focused on the possible hazards of mercury in the environment. The problem became more ominous when Westoo (1966) identified methylmercury (MeHg), the causative agent in the Japanese incidents, in fish from areas supposedly free from MeHg contamination. Westoo’s findings suggested the possibility that other forms of mercury could be transformed into the highly toxic MeHg. This hypothesis was later verified by Wood et al. (1968) who were able to synthesize MeHg with cell-free extracts of methanogenic bacteria, and Jensen and Jernelov (1969) who found MeHg formation in bottom sediments and rotten fish. While evidence has accumulated to show that microorganisms can be a cause of MeHg formation, concurrent attempts to identify the compound in natural waters or sediments have not fared as well. However, various researchers have found that MeHg can also be decomposed biologically. The latter process may help explain why MeHg is seldom found in natural environments contaminated with mercury, but more information is needed to support this hypothesis. The purpose of this research was to evaluate the factors affecting MeHg decomposition and, if possible, to develop a mode to describe the process.