This study investigated the developing metalinguistic capabilities of Hispanic primary school children who are becoming bilingual but whose English reading achievement is below average. Two questions were posed: first, do native- and second-language vocabulary and degree of bilingualism contribute to performance on a metalinguistic task (defining words) and, second, do native- and second-language vocabulary and metalinguistic development at the word level significantly predict reading comprehension in the spring? The results showed that performance on the definition task in English and in Spanish was significantly explained by word knowledge in the language of the task; performance on the definition task in the other language (English or Spanish) but not degree of bilingualism contributed significantly, after the effects of vocabulary in the two languages were accounted for. Both native- and second-language vocabulary and phonological awareness independently contributed to achievement in English reading comprehension. The results suggest that, for children with limited native-language development in the early stages of bilingualism, vocabulary development in both the native and second language and metalinguistic development at the word level may be important education priorities because of their effects on second-language reading comprehension.