The premise that the continued existence of organisations in a democracy depends on both the tacit and the explicit approval and opinions of their many publics is fundamental to public relations theory. Furthermore, the challenge of coping with the potential constraints and opportunities of public opinion as an aspect of the organisational environment is essential to contemporary public relations practice. While the term "public opinion environment" appears intermittently in scholarly and trade publications, the dimensions and characteristics of this aspect of organisational environments remain largely unspecified. This thesis explores two challenges--the first is to conceptualise and measure variation in an important aspect of contemporary organisational environments, the public opinion environment, and the second is to investigate the influence of that environment on the critical and often highly exposed relationships between organisations and activist publics. In suggesting a relationship between variation in the environment and the evolutionary pathways of organisational relationships, the perspective underpinning this thesis is both ecological and evolutionary. Ecological analyses of organisations assume that members of a population are affected similarly by environmental change and share a common dependence on the material and social environment. Consistent with this perspective, this thesis explores the public opinion environment and the organisation-activist relationships therein longitudinally at the population level of analysis. The focal organisational population is Australia's major banks, and the period of interest extends from 1981 to 2001. An evolutionary model of organisation-activist relationships (EOAR) is developed, and the propositions derived from this model are explored using a comparative case study approach. These propositions anticipate and specify associations between variations in dimensions of the public opinion environment of an organisational population and the evolution of organisation-activist relationships in the population. Central to understanding the public opinion environment of an organisational population are the issues around which activist publics organise, public opinion develops, and organisation-activist relationships emerge. Then, the public opinion environment is conceptualised as a set of issues that concern Australia's major banks and their publics. Variations in this "issue set," are described using four dimensions: stability (turnover of issues), complexity (the number of issues in the issue set), intensity (volume of media coverage), and direction (favourability of media coverage for the focal population). To explore the propositions of the EOAR model, I have analysed the variations in these four dimensions in relation to the evolution of organisation-activist relationships. To observe and describe this evolution, I have located the state of these relationships on a conflict continuum using relationship-signalling statements made by organisations and activists and published by the media. Three cases studies from the same organisational population, Australia's major banks, are compared over three different but consecutive seven-year periods from 1981 to 2001. The case studies involved the extensive review of industry reports, submissions and other documents from several government inquiries, and scholarly articles, as well as the content analysis of more than 6, 500 newspaper articles published during each of the three case study periods. The findings of this comparative case study suggest that variations in some dimensions of the public opinion environment of an organisational population are associated with the evolutionary ecology of organisation-activist relationships in that population. However, the associations are more complex than was anticipated by the original propositions of the EOAR model; thus refinements to the model are advanced for further investigation. A primary contribution of this study is that it provides the theoretical apparatus and tools to systematically explore, interpret, and measure variation in the issues comprising the public opinion environment and to track the evolving organisation-activist relationships organised around those issues. Because the population level of analysis and a longitudinal lens are applied, this conceptualisation of the public opinion environment effectively captures and specifies the overlapping and persistent nature of issues. The evidence of this study suggests that when issues have emerged in the public opinion environment at the population level of analysis, they are likely to persist as a hub around which publics organise, providing a focus for discussions and debates for years to come. This persistence, "issue-set inertia," has important implications for organisational relationships. Given these conditions, organisations can monitor and manage their responses to issues. However, it is naïvely optimistic at best to accept the contention of issues management consultants and other pundits that organisations can manage the issues themselves. Furthermore, the findings of this study call into question the value of advice that encourages organisations to deal with issues by seeking to avoid engaging with their activist publics and by downplaying the issues in the media.