A simple model is proposed for studying changes in pathogen populations in response to selection by fungicide and for estimating pathogen fitness. Its implications are discussed in relation to other current models of fungicide resistance. Simulations indicate that the outcome of selection is largely dependent on a fine balance between the relative fitnesses of sensitive and resistant populations and the proportion of host plants treated with the fungicide. Analysis of data on selection by benomyl on Pyricularia oryzae populations indicates that resistance to the fungicide is likely to increase rapidly with moderate use. The model may be extended to describe host selection for increased virulence in pathogen populations. Apart from those that are discontinuously expressed (.e.g adult plant resistance), host resistances may be regarded as behaving like systemic fungicides with maximum persistence. Strategies for limiting the development of resistant pathogen genotypes are discussed.