ABSTRACT: In vivo fluorescence obtained from a Sea Tech fluorometer mounted on SBE CTD of the RIV Ocean Researcher I were calibrated in the spring, summer and autumn of 1993 in the southern East China Sea. Temporal variations ofthe relationship between in vivo fluorescence and chlorophyll a concentrations in seawater were found negligible. In vivo fluorescence (Fre1) is linearly related to chlorophyll a (ChI a) concentrations by: [ChI aJ (mg m') = 0.526(±0.005)*Frel - 0.008(±0.015), R2 = 0.94 This relationship was applied to the in vivo fluorescence profiles that were collected from monthly cruises during the period from August 1990 to July 1991 along two transects northeast of Taiwan. High ChI a distribution was found in November and low in March. The results are compatible with the reported ChI a concentrations from corresponding cruises.