Abstract: A "diagramming debugger" creates a graphical representation of the sequence of messages sent during operation of an object-oriented program. When one object transmits a message to another object, the diagramming debugger displays representations of the transmitting and receiving objects on a computer screen, each representation comprising a box with labels identifying the represented object. The box representing a sending object includes therewithin a symbol (comprising, for example, one or more characters) identifying the method that sent the message, while the box representing the receiving object includes therewithin a symbol identifying the method invoked by the message. The message is represented by an arrow pointing from the symbol identifying the sending method to the symbol identifying the invoked method.