This paper reports on the occurrence of the sei whale (Balaenoptera borealis) in the south-western Atlantic. Only in recent years have sei whale sightings been reported regularly within the surveyed area. In a total of 20 sightings, 65% were reported from August to September. Group size was small, averaging 1.94 (SD = 1.21) individuals per group. During this study, feeding behaviour and interactions with seabirds were observed and are here described. Comparison of our results with historical data from the whaling industry showed that whales were observed where expected based on past catches. Further studies are necessary in order to assess the status of sei whales in the south-western Atlantic. However, our data might indicate a slow recovery in the region. This information will be useful for the conservation of the species in this area and in the southern hemisphere.
