An experimental population of a perennial species, Cassia marilandica, was monitored for 4 years to examine changes in size structure and seed production over time. Plants flowered and set seed the 1st year and annually thereafter. Plant size, seed mass per plant, and reproductive effort were highest in the 2nd year but declined thereafter. Over the first 3 years, density remained constant, but plant size and yield varied. By the 4th year, density, size, and seed yield all declined, owing probably to combinations of drought and insect herbivory. Mean distance to nearest neighbors was consistently positively correlated with plant size but with seed mass in the 2nd and 3rd years only. Plant size was positively correlated with seed mass in each year, with reproductive effort in the first 3 years, but with relative growth rate in the last 2 years only. Population size structure and lifetime reproductive output were generally positively skewed. Except for a slight effect from the 1st to the 2nd year, no cost of reproduction, measured as reduced growth, reproduction, or likelihood of survival was seen. Key words: Cassia marilandica, cost of reproduction, density, Leguminosae, nearest neighbor, perennial, population structure, prairie plants, seed mass.