Dear Marmamers: We're pleased to announce the publication of: Marine Biodiversity Records, page 1 of 6. # Marine Biological Association of the United Kingdom, 2010 doi:10.1017/S1755267210000576; Vol. 3; e68; 2010 Published online On the Occurrence of sei whales, Balaenoptera borealis, in the South-western Atlantic MIGUEL IÑÍGUEZ1,2, JUAN F. MASELLO3, CÉSAR GRIBAUDO4, DÉBORA ARCUCCI1, FLORENCIA KROHLING1 AND JIMENA BELGRANO1 1Fundación Cethus, Potosí 2087, (B1636BUA), Olivos, Prov. Buenos Aires, Argentina. 2Whale and Dolphin Conservation Society, Brookfield House, 38 St Paul Street, Chippenham, SN15 1LJ, United Kingdom. 3 Max Planck Institute for Ornithology, Vogelwarte Radolfzell, Germany. 4 Dirección de Museos de Caleta Olivia, Prov. Santa Cruz, Argentina. e.mail: miguel.iniguez@cethus.org Abstract This paper reports on the occurrence of the Sei whale (Balaenoptera borealis) in the South-western Atlantic. Only in recent years Sei whale sightings have 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 behavior and interactions with sea birds 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 southwestern 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. A pdf copy can be asked to miguel.iniguez@cethus.org Alll the best Jimena Belgrano Fundación Cethus www.cethus.org Argentina