Technical Abstract: The “earthy” and “muddy” off-flavors in pond-reared fish are due to the presence of geosmin or 2-methylisoborneol (MIB) in the flesh of the fish. Similar off-flavors have been reported in fish raised in recirculating aquaculture systems (RAS); however, little information is available regarding the cause of these off-flavors. Our hypothesis was that earthy and muddy off-flavor compounds, previously found in pond-raised fish, are also responsible for off-flavors in fish raised in RAS. In this preliminary study, we examined water, biofilms in RAS, and fillets from cultured arctic charr (known to have off-flavors and requiring depuration) using instrumental (solid-phase microextraction procedure and GC-MS) and human sensory analyses. Geosmin was present in the samples taken from the biofilter and on the side-walls of the tanks. MIB was only found in low levels in the samples. The GC-MS results indicated the presence of geosmin in the fillets (1,095 ng kg-1), but lower levels were found in the water (56.3 ng L-1). Sensory analyses also detected an earthy flavor (i.e., geosmin presence) in the fillets, and, therefore, it appears that geosmin is the main compound responsible for the off-flavor in RAS. Further studies are being performed to identify microorganisms responsible for geosmin production in RAS.