The composition, areal distribution, temporal variation and origin of total suspended matter (TSM) in the Gulf of Trieste (northern Adriatic Sea) in 1991, a year characterized by massive summer macroaggregate formations were studied, in terms of particulate organic and inorganic matter and particulate organic carbon (POC) and nitrogen (PN). The TSM throughout the studied period was characterized by the prevalence of the inorganic fraction except during the period of massive macroaggregates. High surface POC concentrations were observed, especially in the southern part of the Gulf. Generally, POC and PN were poorly correlated to the phytoplankton biomass (Chl. a concentrations) indicating that the particulate in the Gulf of Trieste is probably of a prevalent detrital nature. The principal component analysis of particulate constituents and basic hydrographic parameters (temperature, salinity) showed a net separation of all studied parameters between surface and bottom during the periods of stratified water (late spring and summer). The composition of surface suspended matter was much more variable than that of the bottom. Moreover, the areal distribution of TSM showed lower and more uniform values in the southern part than in the northern part of the Gulf, since the latter.