Due to high biodiversity in tropical rainforests, tree species are aggregatedinto functional groups for modelling purposes. In this article the influencesof two different classifications of tropical tree species into functionalgroups on the output of a rainforest model are analysed. The FORMIND modelis documented. FORMIND simulates the tree growth of tropical rainforests.The model is individual-based and developed from the FORMIX3 model. In themodel, trees compete for light and space in plots of 20 x 20 m in size. Acarbon balance is calculated based on the processes of photosynthesis andrespiration. Using a tree geometry submodel, typical tree variables(e.g. diameter, height, crown length) are calculated. The mortality processis mainly driven by falling trees and the canopy gaps they create. Trees ofthe same functional group and diameter class are represented in one cohort.Simulation results for a primary lowland dipterocarp rainforest in Sabah,Malaysia, are discussed. Detailed structural characteristics of the rainforeststands can be analysed, e.g. the simulation results support the hypothesisthat rainforests grow in a layer structure. A comparison of results for theaggregation of 436 tree species into five or 22 functional groupsrespectively, shows that typical results, such as total stem volume or crownclosure, achieved with five groups are as accurate as simulations with a hugenumber of groups. However, some features such as interspecific competitioncan only be analysed with a huge number (e.g. 22) of functional groups. Themodel has the potential to analyse extinction processes and spatial structureof gap formations in rainforests. The analysis of logging scenarios couldestimate the effects of human impacts in tropical rainforests.