In the Southern Indian Ocean, 2 subspecies of blue whales Balaenoptera musculus spp. occur, the Antarctic B. m. intermedia and the pygmy blue whale B. m. brevicauda. Until the present study was conducted it was assumed that the distribution of these 2 subspecies was delimited by the Antarctic Convergence (52° to 56° S) during the austral summer. Here, we report results from the first year-long, continuous acoustic monitoring of blue whales in mid-latitude waters off the Crozet Islands (46° 25' S, 51° 40' E). Using acoustic data from an International Monitoring System hydroacoustic station, we found that Antarctic and pygmy blue whales occur sympatrically in this area based on detection of their stereotyped calls. Antarctic blue whale calls were recorded year-round, indicating continuous presence in the region, which contradicts the migration paradigm attributed to this subspecies. Three geographically distinct types of pygmy blue whale calls were recorded during the summer and autumn only. The Madagascan call type was the most frequently recorded, while Sri Lankan and Australian call types recorded in this area suggest basin-scale longitudinal and latitudinal movements. During spring and summer, blue whale calls were often associated with higher frequency sounds (D calls), which have been attributed to feeding activity. The summer co-occurrence of blue whale subspecies highlights the importance of this productive sub-Antarctic area as a blue whale hotspot and provides new insights into blue whale seasonal distribution and segregation.