Software development is prone to time-consuming and expensive errors. Finding and correcting errors in a program (debugging) is usually done by executing the program with different inputs and examining its intermediate and/or final results (testing). The tools that are currently available for debugging (debuggers) do not fully make use of several potentially useful visualisation and interaction techniques. This article presents a prototype debugging tool (MVT -- Matrix Visual Tester) based on a new interactive graphical software testing methodology called visual testing. A programmer can use a visual testing tool to examine and manipulate a running program and its data structures. The tool combines aspects of visual algorithm simulation, high-level data visualisation and visual debugging, and allows easier testing, debugging and understanding of software. Categories and Subject Descriptors