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 łAgent Builder,˛ is integrated into the Agentsheets system (providing a substitute for the non-visual AgenTalk editor) 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.