This study describes a vocabulary size test in Arabic used with 339 native speaking learners at school and university in Saudi Arabia. Native speaker vocabulary size scores should provide targets for attainment for learners of Arabic, should inform the writers of course books and teaching materials, and the test itself should allow learners to monitor their progress towards the goal of fluency. Educated native speakers of Arabic possess a recognition vocabulary about 25,000 words, a total which is large compared with equivalent test scores of native speakers of English. The results also suggest that acquisition increases in speed with age and this is tentatively explained by the highly regular system of morphological derivation which Arabic uses and which, it is thought, is acquired in adolescence. This again appears different from English where the rate of acquisition appears to decline with age. While the test appears reliable and valid, there are issues surrounding the definition of a word in Arabic and further research into how words are stored, retrieved and processed in Arabic is needed to inform the construction of further tests which might, it is thought, profitably use a more encompassing definition of the lemma as the basis for testing.