In this paper, we present a metamodel for textual use case descriptions, structurally conforming to the UML, to specify the behavior of use cases in a flow-oriented manner. While being primarily targeted at supporting requirements engineers in creating consistent use case models, the metamodel defines a textual representation of use case behavior that is easily understandable for readers, who are unaware of the underlying metamodel. Hence, the known benefits of natural language use case descriptions are preserved. Being formalized, consistency between UML-based use case representations and their textual descriptions can be automatically ensured. With NaUTiluS we present an extensible, Eclipse-based toolkit, which offers integrated UML use case modeling support, as well as editing capabilities for their textual descriptions.