Over a period of seven seasons (1995–2001 inclusively) four distinct epidemics of the rust pathogen Melampsoridium betulinum were recorded in stands of Betula pubescens. During those epidemics, host plants incurred varying levels of disease severity. Some individuals suffered high levels on all occasions; some low; while yet others were either intermediately affected or showed variable disease severity from season to season. Tests of pathotypic differences among 33 isolates of M. betulinum collected from six sites found a broad range of pathotypes ranging from highly virulent to highly avirulent. Similarly a random sample of 40 B. pubescens lines from one site showed a wide range of resistance phenotypes, although individuals that were either susceptible to all but one pathotype or resistant to all pathotypes were commonest. A strong relationship existed between the cumulative level of disease incurred by B. pubescens individuals in the field and the mean susceptibility of each host line as determined by their reaction to infection by each of the 33 different isolates of M. betulinum individually. Resistance in this B. pubescens population to M. betulinum is postulated to be based on a mixture of quantitative and qualitative traits, selection for which has resulted from an interplay of life history attributes of both host and pathogen.