Mugu Lagoon is an estuary in southern California, listed as impaired for mercury. In 2005, we examined mercury cycling at ten sites within at most four habitats. In surface water (unfiltered and filtered) and pore water, the concentration of total mercury was correlated with methylmercury levels (R2=0.29, 0.26, 0.27, respectively, p<0.05), in contrast to sediments, where organic matter and reduced iron levels were most correlated with methylmercury content (R2=0.37, 0.26, respectively, p<0.05). Interestingly, levels for percent methylmercury of total mercury in sediments were higher than typical values for estuarine sediments (average 5.4%, range 0.024-38%, n=59), while pore water methylmercury Kd values were also high (average 3.1, range 2.0-4.2l kg(-1), n=39), and the estimated methylmercury flux from sediments was low (average 1.7, range 0.14-5.3ng m(-2) day(-1), n=19). Mercury levels in predatory fish tissue at Mugu are >0.3ppm, suggesting biogeochemical controls on methylmercury mobility do not completely mitigate methylmercury uptake through the food web.