Methyl mercury (Hg) was determined in rainbow trout (Oncorhynchus mykiss) and organisms in the lower tropic levels: smelts (Retropinna retropinna), bullies (Gobiomorphus cotidianus), koura (Paranephrops planifrons); and zooplankton (Daphnia carinata and Calamoecia lucasi) in Lakes Okareka, Okaro, Tarawera, Rotorua and Rotomahana, New Zealand. Water concentrations of total Hg (Hg_T) and methyl Hg were also measured. Mean methyl Hg concentrations in the trout, the prey species (smelts, bullies and koura) and zooplankton increased linearly with mean Hg_T and methyl Hg chloride (CH₃HgCl) concentrations in water. Most of the bio-magnification of methyl Hg occurred in the lower trophic levels of the trout food web (10^4.72) between the zooplankton and water. The bioaccumulation factors between the forage fish and zooplankton were 10^0.73 for bullies and 10^1.06 for smelt. Methyl Hg was 10^0.41 to 10^0.95 times greater in the trout then their prey.