Abstract: As has been the case internationally, deinstitutionalisation of dual diagnosis (intellectual disability and mental illness) services has also occurred in New Zealand. Inpatient services have been redefined to respond to the more acute focus that has arisen out of this deinstitutionalisation process and nurses are having to redefine their roles in response. This study was undertaken to explore and describe the culture of nursing practice in a dual diagnosis inpatient unit in one psychiatric hospital. A focused ethnographic approach was used to triangulate data gathered from fieldwork observations, review of documents and semi-structured interviews. Schein's (1985) levels of culture model, was used to identify and explore the artifacts, values and assumptions evident in this nursing practice. Analysis presents three key themes categorised as 'communication', 'assessment' and 'safety'. While these key themes are shown to be evident in the everyday practice of the nurses, how these relate to the notion of 'dual diagnosis nursing' is not clear. Therefore, the author describes the major finding of this study as revealing a nursing culture holding tight to traditional psychiatric and psychopaedic nursing practices and struggling to develop a distinctive culture in the absence of a defined dual diagnosis knowledge base. The author concludes that these findings suggest an urgent need to provide nurses with support in gaining contemporary knowledge regarding dual diagnosis nursing. Support for nurses in advancing these areas then impacts on support for the patients. It is suggested that additional research is undertaken to assess the learning needs of the nurses in order to develop clinical practice guidelines for this area. Further recommendations are made to address system issues which are contributing to the gap in knowledge.