The horizontal and vertical distribution of marine humic-like fluorophore, namely, chromophoric dissolved organic matter (CDOM) fluorescence, was surveyed in Ise Bay, Japan. The distribution patterns of salinity and marine humic-like fluorescence intensity suggested that riverine humic-like fluorophore conservatively distributed along with fresh-sea water mixing in surface water at Ise Bay. However, analysis of mixing behavior of riverine CDOM implied that the 25.1 ¿ 10.6% in average of bulk marine humic-like fluorescence intensity of Ise Bay surface waters was derived not from a terrestrial origin but an in situ production. A degradation experiment using natural plankton demonstrated the rapid production of marine humic-like fluorophore within a day. The results consistently suggested that in situ production of marine humic-like fluorophore plays an important role in the dynamics of CDOM in coastal environments.