Cross-sectional field and experimental studies were carried out to determine the mercury distribution in the Mabubi River and to establish its biological effects in the ecosystem using snails as bioindicators. The median and range for total mercury concentration in Lymnaea stagnaliswere 4298 and 12.62 to 50618 respectively. The total mercury concentrations were recorded as below the detection limit (<0.01μg/g) in 28 snails.Experimental exposure studies using sub-lethal concentrations of mercury (II) chloridewerecarried out on Lymnaea stagnalis. The hatchability and growth of L. stagnalis after 24, 48, 72 and 96 hours of exposure at 56 μg/L and 112 μg/L of HgCl2 were significantly reduced (P <0.05) during the four weeks of observations. Effects of mercury were apparent from the first week of the exposure. The hatching success for the control and exposed groups were 98.7% and 29-60% respectively and the difference was significant (P <0.05). The growth coefficient was higher (818.56%) in control group than in those exposed to high Hg concentrations (56μg/L and 112μg/Lat 600% and 505.81% respectively).No significant effect (P >0.05) was observed on fecundity and survival of snails between control and exposed groups. The results from this study show that mercury waste is present in the study area and is carelessly released into the environment. Findings further demonstrate the usefulness of L. stagnalis as a bioindicator for monitoring mercury pollution in aquatic ecosystem. Environmental education to the artisanal gold miners, law enforcement and strict regulatory measures are important in order to avoid health and ecological consequences arising from the continual release of mercury.Lake Victoria Research Initiative(VICRES)