Social signals indicating the emotional state of the signaller are described as emotive signals. Their structure is often variable and graded. Emotive acoustic signals are associated with close proximity of animals often in combination with tactile and visual communication. By contrast, signals used for location and identification of conspecifics are mainly stereotyped and discrete. As yet, research on killer whale acoustic communication has focused mainly on discrete calls. Whistles, characterized by variable pure tones, are poorly investigated in killer whales and their function in communication remains unclear. In this study we classified whistles from different killer whale pods and examined whistle emission during different behavioural contexts. Simultaneous underwater recordings and surface behavioural observations were made on resident killer whales in the Johnstone Strait, British Columbia, in summer 1994. Whistles were analysed using a Real Time Spectrographic Analyser. Every acoustical clan of killer whales seem to produce whistles with some distinctive features. Whistles are produced more often during socialising than during any other behavioural category (foraging, travelling, resting, beach rubbing). During socialising killer whales form a group often in close body contact and engage in a variety of social displays: breaches, chasing, rolling over each other, and sexual behaviours. In this behavioural context whistles might serve as emotive signals indicating the emotional state of the signalling animal.