Vesicular-arbuscular mycorrhizal infection and morphology was measured in six co-existing plant species of a semi-natural grassland during a two year period in order (1) to measure within-year variation in mycorrhizal infection (2) to determine whether seasonal trends in mycorrhizal infection were the same in both years and (3) to quantify and interspecific differences in this variation. Seasonal trends in total mycorrhizal infection in the six plant species grouped together were very different in the two years. When the plant species were considered individually distinct patterns emerged. Mycorrhizal infection in Plantago lanceolata L.; Rumex acetosa L. varied greatly both within and between years. In contrast Festuca rubra L. Holcus lanatus L. Lathyrus pratensis L.; Trifolium pratense L. all exhibited more constant levels of infection within one year and between successive years. However interspecific differences in infection occurred between these four species. Although there was temporal variation in hyphal density arbuscule and vesicle density varied very little with time and thus could not be used to indicate whether there are predictable periods of host cost or benefit. Vesicle width varied significantly between the plant species; this might represent infection by different fungal species.