Cadmium (Cd) is produced mainly as a by-product of zinc mining. In Thailand, the largest zinc mine is located in the Mae Sot district, Tak Province. Samples of Monopterus albus were collected from paddy fields in 4 sites, three downstream and one upstream from the zinc mine. The upstream site was considered to be uncontaminated while the three downstream sites were considered to be contaminated with Cd. Studies on the accumulation level of cadmium were conducted on the liver of the fish using the atomic absorption spectrophotometer technique. The metallothionein (MT) gene expression level in the liver, as a potential biomarker for long-term Cd exposure in their natural habitat, was also assessed. The level of hepatic MT gene expression was performed by quantitative real-time PCR. The result showed that Cd accumulation in the liver was much higher in swamp eels collected from the downstream sites when compared to those collected from the upstream site. The hepatic MT level in the upstream site was 0.75-fold, while the other three downstream sites were 0.36-, 4.44- and 0.94-fold. There is no parallel correlation between hepatic cadmium levels and hepatic MT gene expression. This study then suggests that MT gene expression biomarkers might be not suitable for swamp eels with prolonged exposure to Cd.