Abstract. It is claimed in [45] that first-order theorem provers are not efficient for reasoning with ontologies based on description logics compared to specialised description logic reasoners. However, the development of more expressive ontology languages requires the use of theorem provers able to reason with full first-order logic and even its extensions. So far, theorem provers have extensively been used for running experiments over TPTP containing mainly problems with relatively small axiomatisations. A question arises whether such theorem provers can be used to reason in real time with large axiomatisations used in expressive ontologies such as SUMO. In this paper we answer this question affirmatively by showing that a carefully engineered theorem prover can answer queries to ontologies having over 15,000 first-order axioms with equality. Ontologies used in our experiments are based on the language KIF, whose expressive power goes far beyond the description logic based languages currently used in the Semantic Web.State-of-the-art theorem provers for first-order logic (FOL) are highly sophisticated and efficient programs. Moreover, they are very flexible tools and can be tuned to a number of applications. For example, Vampire [35] provides a large collection of parameters that can be used to give better performance for various classes of applications. In addition, Vampire implements a number of literal selection functions and internally contains a library for defining such functions in a simple way; this makes it possible to simulate various proof-search algorithms and even provide decision procedures for decidable classes of first-order logic (see, e.g., [23]).However, there was a common belief that provers like Vampire cannot directly be used for efficient reasoning with very large ontologies using expressive languages such as KIF [14] for two reasons. Firstly, these provers are optimised for reasoning with relatively small axiomatisations. Secondly, they do not support some extensions of FOL required in KIF.In this paper we describe an adaptation of Vampire to support reasoning for expressive ontology languages and present experimental results which show that it can be used for efficient reasoning with large ontologies using extensions of the first-order language.The authors are partially supported by grants from EPSRC. This paper is structured as follows. In Section 1 we briefly overview expressive languages for ontologies, including the language KIF, and FO provers. In Section 2 we describe the adaptation of Vampire for reasoning with large ontologies. For our experiments we selected ontologies implemented in KIF since they offer a high degree of sophistication compared to ontologies using Description Logic (DL) based languages, and there are publicly available KIF-based ontologies containing thousands of FO formulas with equality. However, our adaptation is quite general, and could be used for other expressive ontology languages.Note that there is no way to compare Vampire with description logic prove...