The creation of individual 3D models to include within a virtual world can be a time-consuming process. The standard approach to streamline this is to use procedural modeling tools, where the user adjusts a set of parameters that defines the tree. We have designed Gen- Tree, an interactive system that uses a genetic algorithms (GA) approach to evolve procedural 3D tree models. GenTree is a hybrid system, combining the standard parameter adjustment and an interactive GA. The parameters may be changed by the user directly or via the GA process, which combines parameters from pairs of parent trees into those that describe novel trees. The GA component enables the system to be used by someone who is ignorant of the parameters that define the trees. Furthermore, combining the standard interactive design process with GA design decreases the time and patience required to design realistic 3D polygonal trees by either method alone.