Abstract
This paper examines a recent trend amongst software agent application and platform developers to desire the ability to send domain-specific objects within inter-agent messages. If this feature is to be supported without departing from the notion that agents communicate in terms of knowledge, it is important that the meaning of such objects be well understood. Using an object-oriented metamodelling approach, the relationships between ontologies and agent communication and content languages in FIPA-style agent systems are examined. It is shown how object structures in messages can be considered as expressions in ontology-specific extensions of standard content languages. It is also argued that ontologies must distingish between objects with and objects without identity.