Dear colleagues, On behalf of my co-authors, I’m pleased to share our recently published paper in *Marine Ecology Progress Series*, which investigates the movements of the endangered sei whale (*Balaenoptera borealis*) in the Southwestern Atlantic Ocean. ABSTRACT The sei whale *Balaenoptera borealis* was once abundant in the Southwestern Atlantic Ocean (SWA); however, the species was severely depleted by whaling and is now Endangered. While some occurrence data exists for sei whales in the SWA, understanding of this species’ population ecology remains limited. Here, we used visual surveys and satellite telemetry to investigate sei whales’ occurrence and movements in Santos Basin (southeastern Brazil). Between 2016 and 2024, 119 sei whale groups were sighted (1-32 ind.; mean: 3.4 ± 4.1). Although the Brazilian coast is a presumed breeding ground, only 3 calves were recorded. In total, 26 sei whales were tagged, with track durations of 3-53 d (mean: 22.5 ± 16.9). Total distance traveled ranged from 112.6 to 4313.7 km (mean: 1522.1 ± 1179.5). A hidden Markov model was fitted to telemetry data to predict behavioral states; models showed that sei whales predominantly engaged in area-restricted search (ARS) (53.6%) and transiting behavior (46.4%). Most locations estimated as ARS were near topographic features such as submarine canyons along the continental slope and close to seamounts. Occupancy time indicates frequent annual use of these areas, but movement patterns were not predictable; nevertheless, they provide important information on use of breeding grounds. ARS occurred in productive areas and may be related to opportunistic feeding, but more research is needed to describe this species’ behavior in tropical breeding grounds. We provide a robust data set, showing the regular occurrence and movements of sei whales in the pelagic waters off southeastern Brazil during winter and spring. The paper can be found at: https://doi.org/10.3354/meps14878 With kind regards, Clarêncio Baracho