For the Australian sea lion (Neophoca cinerea), an endangered keystone predator that demonstrates high rates of pup mortality and limited population recovery, an understanding of the role of infectious disease in influencing pup health, and how it may contribute towards shaping population demography, is a key knowledge gap. This thesis investigated the taxonomy, epidemiology, clinical impact, and management of hookworm infection in N. cinerea to address the hypothesis that hookworm infection is a significant cause of disease and mortality in this species. Hookworms collected from N. cinerea pups were identified and described as a novel species (Uncinaria sanguinis). Transmammary transmission in the immediate post-parturient period was implicated as the predominant route leading to patent hookworm infection in pups; however, in contrast to the fundamental role that colony substrate appears to play in shaping the epidemiology of hookworm infection in other otariid hosts, this thesis determined that all N. cinerea pups are infected with U. sanguinis irrespective of the type of colony substrate and that the intensity of hookworm infection appears to be influenced by colony-specific seasonal differences in host behaviour. The clinical impact of hookworm infection in pups was quantified and the occurrence of seasonal patterns in health parameters and the magnitude of colony pup mortality were related to the dynamics of hookworm infection. In addition, the effectiveness of ivermectin to eliminate hookworm infection was investigated. This thesis determined that U. sanguinis is an important cause of disease and mortality in N. cinerea; this thesis contributes towards an improved understanding of the role of infectious disease in influencing the health status and population demography of this endangered species, informing conservation management and providing a solid foundation for further investigations of the effect of disease on the health status of free-ranging species.