This paper describes the issues involved in extending a trans-lingual lexicon, the TextWise Conceptual Interlingua (CI), with Arabic terms. The Conceptual Interlingua is based on the Princeton English WordNet (Fellbaum, 1998). It is a central component in the cross-lingual information retrieval (CLIR) system CINDOR (Conceptual INterlingua for DOcument Retrieval). Arabic has a rich morphological system combining templatic and affixational paradigms for both inflectional and derivational morphology. This rich morphology poses a major challenge to the design and building of the Arabic CI and also its validation. This is because the available resources for Arabic, whether manually constructed bilingual lexicons or lexicons automatically derived from bilingual parallel corpora, exist at different levels of morphological representation. We describe here the issues and decisions made in the design and construction of the Arabic-English CI using different types of manual and automatic resources. We also present the results of an extensive validation of the Arabic CI and briefly discuss the evaluation of its use for CLIR on the TREC Arabic Benchmark collection.