This study aimed to determine the concentrations of plasma methylmercury (Me-Hg) and inorganic mercury (I-Hg) in a population exposed to Me-Hg. In addition, associations between each form of mercury (Hg) and gender, age, plasma selenium (Se), and oxidative stress markers were also investigated. The mean plasma I-Hg level was 5.7 μg/L while the mean for plasma Me-Hg was 3.6 μg/L, representing approximately 59 and 41% of the total Hg in blood, respectively. However, several plasma samples contained higher percentages of Me-Hg. Age displayed a direct linkage with plasma I-Hg levels, whereas gender did not correlate with any of the Hg species. In addition, fish intake was only correlated with and a predictor of plasma Me-Hg, suggesting that plasma I-Hg levels originated endogenously through a demethylation reaction that needs to be verified. Further, plasma Me-Hg was markedly correlated with adverse effects to a greater extent than plasma I-Hg and may be considered a valuable, reliable internal dose biomarker for Hg in chronically Me-Hg- exposed individuals.