Hector's dolphins are threatened with local extinction by entanglement in coastal gillnets. This thesis provides data on population biology, social organization and behaviour of Hector's dolphins that help assess human impacts on their populations. To estimate population growth, I integrated anatomical studies which estimated longevity and age at first reproduction, with photographic field studies which estimated reproductive rate and survival rate. Sixty incidentally caught and beach-cast dolphins were aged from the growth layers in their teeth. Maximum age was 19 years for females and 20 for males. Females gave birth to their first calf at 7 to 9 years old, and thereafter had one calf every 2 to 3 years. Population models using these data predicted maximum population growth rates of 1.8 to 4.4% per year. These rates were exceeded by the number of Hector's dolphins recently killed in gillnets in the Pegasus Bay-Canterbury Bight area. Survival rates (including gillnet mortality) estimated using photographic identification, also suggested that this population was unable to cope with recent gillnet entanglement levels. Population models using these survival rates (0.797 to 0.865 after the first year of life), even in combination with the most optimistic reproductive rates, resulted in a decreasing population. The population models were also used to explore the likely consequences of management strategies which reduce entanglement mortality. Population and population growth rate fluctuated markedly for several decades after a significant reduction in entanglement mortality, especially if the age structure of the population was biased towards younger individuals. A study of social organization and behaviour pointed to another potential conservation risk. The social organization of Hector's dolphins was studied using photographic identification. Each individual associated loosely with a relatively large number of others, rather than with a few close associates, and groups frequently joined other groups and exchanged members. Sequence analysis was used to classify Hector's dolphin behaviour into five categories: 'feeding', 'sexual" 'aggressive', 'play' and 'aerial', using behaviour sequence analysis. The number of sexual behaviours per individual was highest in groups of 11-15 dolphins, and tended to increase after groups came together. The fluid association patterns and increase in sexual behaviours after groups come together suggest that Hector's dolphins have a promiscuous mating system in which males search for rather than monopolize females. Such a mating system has the potential to reduce fertilization rates in areas of low abundance.