AbstractGroups ofEubalaena australisin the Gulf of San Jose, Argentina were characterized and classified. A one‐zero sampling method was used for collecting data. A distance matrix was calculated with the 124 groups recorded and 10 characters including composition, size, activities, belly‐up and tail‐up positions, flipper and tail slaps, and breaching. Cluster analysis separated groups into 12 types. Six low‐interaction group types were differentiated primarily by their composition, and extent of swimming and resting behaviors. Six interaction group types were characterized by the presence of whales in the belly‐up position, by group size, by the absence of females with calves, and by sexual activity. Some interpretations of the possible functional meaning of the observed group types are