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Social organization of dolphins in extensive societies has not been well studied. Off Kaikoura, New Zealand, thousands of dusky dolphins (Lagenorhynchus obscurus) gather, feeding nocturnally on deep scattering layer prey, resting and socializing diurnally. During 1997-2003, interval sampling was used to monitor large assemblages numbering hundreds (n=169), smaller mating groups (mean+s.e.=7+1.6 adults, n=42), mother-calf nurseries (mean+s.e.=13+1.6 adults, 1+0.5 juveniles, 4+0.7 calves and 1+0.4 neonates, n=41), and non-mating adult groups (mean+s.e.= 9+1.3 adults, 1+0.2 juvenile, n=37). Group size, distance from shore (east), ranging along shore (north), traveling, inter-individual distance, and noisy leaping peaked in winter (n=39), with dolphins maintaining closer proximity to each other in smaller, more restful groups, closer to shore during the spring-summer-autumn (n=234) reproductive seasons. Dolphin groups were found closest to shore (west) during early morning, spread out and leaping often. Resting peaked at midday in tight groups. Late in the day, dolphins spread out, moving eastward (offshore) in preparation for feeding. Large groups exhibited coordinated travel, with noisy leaps as a directional iv signal. “Mating of the quickest” occurred in groups of (median) 6 males chasing 1 female. Leaping rarely occurred in restful nurseries, which at times associated with Hector’s dolphins (Cephalorhynchus hectori). Other mixed-species groups included common dolphins (Delphinus delphis), southern right whale dolphins (Lissodelphis peronii), long-finned pilot whales (Globicephala malaena), and bottlenose dolphins (Tursiops truncatus). Killer whales (Orcinus orca) elicited predator assessment and evasion. Whale riding occurred with larger whales. Residence was seasonal, with 1,969+814.9 from a population of 12,626 dolphins spending 103+38.0 days in Kaikoura (mean+s.e., mark-recapture mortality, single-season lagged-ID emigration models, n=153 weeks). Dolphins (n=39) summering in Kaikoura migrated to the Marlborough Sounds in winter, where small, coordinated groups foraged diurnally on schooling fishes in shallow bays, often associated with sea birds and New Zealand fur seals (Arctocephalus forsteri). Aquaculture may threaten dusky dolphin foraging habitat in Admiralty Bay, where an estimated 220 dolphins gathered to feed each winter. Photo-identification research, enhanced by digital techniques, demonstrated a structured fission-fusion society. Dusky dolphins associated with preferred long-term (>1,000 days) hunting companions in Admiralty Bay and non-random casual acquaintances (200 days) in Kaikoura (lagged-association models).