The biogeochemical cycling of dimethylated sulfur compounds was investigated in the Saguenay Fjord during June 1996, just prior to the flooding in the catchment area. Dimethylated sulfur compounds, namely dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP), and dimethylsulfoxide (DMSO), were determined at six sites. In comparison with other coastal environments, including the Gulf of St. Lawrence, DMS (0-1.8 nmol·L-1, average <0.2 nmol·L-1) and DMSP (0-23 nmol·L-1, average <0.8 nmol·L-1) concentrations were relatively low because the brackish waters were poor in algal abundance and dominated by species not noted as major DMS and DMSP producers. Similarly, dissolved DMSO levels (<0.016 nmol·L-1) were low due to the limited availability of its DMS precursor and the shallow photic zone. Particulate DMSO was measured (0-110 nmol·L-1, average ~8 nmol·L-1) and occasionally exceeded particulate DMSP concentrations (0.016-140 nmol·L-1, average ~3.5 nmol·L-1). Given the low dissolved DMSO concentrations, the presence of particulate DMSO provides evidence for its direct biosynthesis by phytoplankton.