ABSTRACT
The aim of this work was to analyze the size and spatial structures and demography of Actinostemon concolor (Spreng.) Müll. Arg. (Euphorbiaceae) in the flooded areas of Mata dos Godoy State Park. Fifty plots of 100 m² were established and three surveys were conducted. All the individuals were measured, mapped and divided into categories as seedlings, juveniles and adults. The coefficient of skewness, L-function of Ripley, and the matrix population models were used in the study. The population showed positive coefficient of skewness and the individuals were distributed in groups. The population growth rates (λ) were equal or lower than 1.0 and the adult survival and recruitment of juvenile to adult were the vital rates that most affected the values of λ. Considering these results and the unpredictability of the environment, it could be concluded that the individual numbers of A. concolor would fluctuate over time, but the species would remain as one of the most important in the Park.