The fluorescence of filtered seawater and porewaters from the Santa Barbara Basin has been measured. Porewater fluorescence exceeded that in the overlying seawater by a factor of 10--500. The water column profile of fluoroescence can be explained by photochemical degradation of fluorescent components in surface waters and diffusion of ''marine humic substances'' from the sediments into basin bottom waters. Porewater fluorescence increases linearly with depth to at least 2 m and correlates well with titration alkalinity and ammonium ion concentration. The sources and the type of molecules responsible for fluorescence remain unknown.