Dissolved and particulate Th-234 activities were determined and phyto- and zooplankton biomass were periodically measured 8 miles off Nice (Mediterranean Sea) during spring 1987. The results show a strong variability of Th-234 distribution on short time scales in northwestern Mediterranean surface waters. The good correlation observed between the zooplankton biomass and the rate of Th-234 export to deep water in particulate form is in agreement with the assumption that the residence time of particulate Th-234 in oceanic surface waters is controlled by zooplankton grazing. Moreover, our results indicate the importance of salps in particular as efficient removers of small suspended particles in surface waters.