Based on our previous work on the formal specification language SLABS as well as a methodology and modelling language for modelling and specifying multi-agent systems, we further investigate how diagrammatic models of multi-agent systems can be used to derive formal specifications in SLABS. The modelling language is further developed by introducing behaviour diagrams and scenario diagrams for modelling agents’ behaviour in various scenarios of their environment. It supports derivation of the specifications of agents’ behaviour through scenario analysis. The language and its usage are illustrated by an example of the evolutionary multi-agent ecosystem Amalthaea developed at MIT Media Lab.