The concentrations of total and particulate organic carbon in the waters of Fanning Island Lagoon are high and fairly uniform throughout, averaging 1.68 mg/liter and 80 µg/liter, respectively. Phytoplankton and reefs in the lagoon produce 7.8 X. 10^4 kg of organic carbon daily, giving a turnover time of only 11 days for organic carbon. Only 0.4 percent of the daily production is lost from the lagoon by tidal exchange, the remainder is respired by organisms in the lagoon. Fanning Lagoon is a rich and unique environment which, because of its low flushing rate of about 230 days, has little effect on the surrounding ocean.