Traditional theories of Semitic morphology hold that two abstract morphemic units, the root (e.g., "k-t-b") and the word pattern (e.g., "-i-aa-"), are the basis for word formation (e.g., "kitaab" [book]). Previous lexical processing studies in Hebrew and Arabic have confirmed the independent morphemic status of these two units, and the root in particular, whose existence was disputed under word-based theories. To further this line of investigation, the current study asks: (1) Is root priming in Arabic caused by form and semantic overlap or is it independent of these two factors? (2) Does the Arabic lexicon impose linearity constraints on the root consonants? (3) Does higher proficiency in a second language (English) have any consequences for lexical organization and processing in the first language (Arabic) of bilingual speakers? In addressing these research questions (RQs), a masked priming experiment was designed using a within-target design. To get at RQ1, the priming effects between same- root word pairs with both decreased semantic and form overlap were compared to the priming between orthographic minimal pairs. With respect to positional coding constraints (RQ2), this study included transposed-letter (TL) existing words. RQ3 was investigated by comparing lexical processing in native speakers of Arabic who are highly proficient in English to processing in near monolingual speakers of Arabic. Results show that for near monolingual speakers of Arabic, there were significant priming effects between same-root words and no orthographic or TL priming effects. In contrast, native speakers of Arabic with high proficiency in English show numerical priming in the TL condition and no priming in the root condition. The results of the monolinguals are consistent with the morpheme-based/ decompositional view of Semitic morphology. On the other hand, the results of the bilinguals extend previous research on the effects of the second language on the first to the domain of lexical processing. The implications of these results are discussed. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]