A model of infection of roots by vesicular--arbuscular mycorrhizas, presented earlier (Buwalda et al., 1982), is used to study the effects of phosphorus on the spread of infection in root systems. Two hosts, with contrasting responses to mycorrhizal infection (wheat and leek), were grown at a range of phosphorus levels, and sequential measurements of growth and concentrations of phosphorus in plants, and infection of roots, were made. Increasing levels of phosphorus in the soil increased concentrations of phosphorus in the plants and reduced the percentage of root length infected in both hosts, but the effect was greater in wheat. Fitting the model showed that the inherent rate of spread of the fungus was not significantly altered by applications of phosphorus. However, the maximum proportion of the root system which could become infected was reduced, especially in wheat. Some implications of this finding on our understanding of the mycorrhizal symbiosis are discussed. [ABSTRACT FROM AUTHOR]