Insecticide (Malathion-60) was applied to replicated plots in a calcicolous grassland establishing naturally on a field abandoned from cereal cultivation 5 years ago. Comparison with matched control plots showed that both the direction and rate of plant succession over a 2 year period were influenced by natural levels of insect herbivory. The insecticide had no independent effect on plant growth. Insect herbivores affected succession by different effects on plant life-history grouping. For example, in insecticide-treated plots the growth of perennial grasses was enhanced and especially the vigorous species such as Agropyron repens and Agrostis stolonifera. Common short-lived perennial herbs, such as Medicago lupulina, were defoliated and died in control plots. The net result was a taller, denser sward in insecticide-treated plots, with lowered plant diversity by the end of the second season (1986). D-Vac sampling showed that the insects responsible were mainly sucking insects (e.g. Auchenorrhyncha) on the grasses and chewing Coleoptera (Curculionoidea and Chrysomeloidea) on the short-lived perennials. The results are contrasted with similar experiments on acid grassland where an earlier invasion of perennial legumes and a different insect fauna resulted in apparently simpler effects on the rate of succession.