This study investigated the effects of first language word-level reading skills on the development of English as a second language (ESL) word-level reading skills. A crosslinguistic analysis indicates that native Arabic and Japanese speakers are likely to encounter different types of ESL word-level reading difficulties. Specifically, native Arab speakers are likely to exhibit difficulties with prelexical ESL word recognition processes, whereas native Japanese speakers are likely to exhibit difficulties with on-line ESL word integration processes that integrate words into phrase/clause structures for comprehension. Results from a lexical decision task showed that a group of Japanese ESL learners had significantly faster and more accurate word recognition skills compared to a proficiency-matched Arab ESL group. In contrast, both groups read words within sentences in a sentence reading task at the same speed, though the Arab ESL group was significantly more accurate in integrating words into larger phrase and clause units and comprehending them than the Japanese ESL group. These results indicate that Arab and Japanese ESL students have different word-level reading difficulties, implicating different learning needs and pedagogical interventions for developing ESL reading proficiency.