This paper presents a tool called TUGEN which is used for automatic test suite derivation from formal protocol specification. TUGEN is based on a formal model called EBE (External Behavior Expression) which can be obtained from formal protocol specification in either Estelle of LOTOS. This model specifies only the external behavior of a protocol in terms of the input/output sequences and their logical (function and predicate) relations. Based on the EBE specification of a protocol, a test sequence derivation method is used to identify associations between inputs and outputs through the interaction paths and their I/O subpaths, then generic test cases specified in TTCN (Tree and Tabular Combined Notation) can be generated from these I/O subpaths. Comparison of test cases generated from this tool and those in ISO/IEC DIS 8882 part 2 for X. 25 LAPB protocol shows that the resulting set of test cases of TUGEN is concise and effective. It is our belief that TUGEN can be a powerful utility for protocol test suite generation.