We present a new approach to conformance testing of black-box reactive systems. We consider system specifications written as linear temporal logic formulas to generate tests as sequences of input/output pairs: inputs are extracted from the Buchi automata corresponding to the specifications, and outputs are obtained by feeding the inputs to the systems. Conformance is checked by comparing input/output sequences with automata traces to detect violations of the specifications. We consider several criteria for extracting tests and for stopping generation, and we compare them experimentally using both indicators of coverage and error-detection. The results show that our methodology can generate test suites with good system coverage and error-detection capability.