This paper describes PEGASUS, a spoken language interface for on-line air travel planning that we have recently developed. PEGASUSleverages off our spoken language technology development in the ATISdomain, and enables users to book flights using the American Airlines EAASYSABREsystem. The input query is transformed by the speech understanding system to a frame representation that captures its meaning. The tasks of the System Manager include transforming the semantic representation into an EAASYSABREcommand, transmitting it to the application backend, formatting and interpreting the xesulting information, and managing the dialogue. Preliminary evaluation results suggest that users can learn to make productive use of PEGASUSfor travel planning, although much work remains to be done.