A visual programming facility is described which is based on a visual rule formalism. It permits programmers to
specify the behaviors of agents in the class of program objects in the Agentsheets system of Repenning. The
resulting agents can be used in designing simulations for two-dimensional cellular worlds such as in automobile
traffic flow situations, video games, and cellular automata studies. The programming facility, called "Agent
Builder", is integrated into the Agentsheets system in such a way that agent programming becomes a process of
arranging icons in a worksheet, using a special gallery of icons. Several applications of Agent Builder are
presented, and its limitations and possible extensions are discussed.