In this paper, we present a feature-oriented requirements modelling language (FORML) for modelling the behavioural requirements of a software product line. FORML aims to support feature modularity and precise requirements modelling, and to ease the task of adding new features to a set of existing requirements. In particular, FORML decomposes a product line's requirements into feature modules, and provides language support for specifying tightly-coupled features as model fragments that extend and override existing feature modules. We discuss how decisions in the design of FORML affect the evolvability of requirements models, and explicate the specification of intended interactions among related features. We applied FORML to the specification of two feature sets, automotive and telephony, and we discuss how well the case studies exercised the language and how the requirements models evolved over the course of the case studies.