ECOLOGICAL models describe the conditions required for chaotic population dynamics1-7, but there are few studies with which to test these predictions4,8-10, and none for perennial plants. In a five-year experiment, a perennial grass exhibited numerous traits associated with chaotic dynamics. Biomass oscillations were greater on more productive soils, with plots on the richest soils exhibiting a 6,000-fold crash. Curves relating the biomass one year to that in the previous year had the peaks and steep slopes associated with chaos, and the dependence of plant biomass on productivity became fuzzy over time. These dynamics resulted from the time-delayed inhibitory effect of plant litter on subsequent growth. A model incorporating litter inhibition predicts oscillations and chaos as productivity increases.