Upper ontologies are sophisticated systems that require an expressive language to be properly formalized and correctly implemented. This paper provides a formal study of one of these ontologies, called YAMATO, by providing an axiomatization in first-order logic of part of the main system. YAMATO, which has been available in OWL for some years and is used in research projects as well as in applications, is quite rich in terms of categories and relations. The system is also interesting from its ontological perspective as it presents a different combination of ontological choices with respects to todays popular upper ontologies. Here we isolate a fairly compact fragment of this system that covers important categories, such as Process and Role, and relations, such as Enacting and CausallyContributing. The axiomatization is a first step towards the full exploitation of YAMATO in information and computational systems.