This study investigated whether weakness in using morphological analysis to infer new word meanings during reading is a source of poor text comprehension and the relative importance of psycholinguistic and cognitive factors as contributors of poor text comprehension in English-only and English language learners. Thirty-seven poor comprehenders and 44 typical readers in fourth and fifth grades in the United States were identified. Students were assessed on incidental word learning, vocabulary, morphological awareness, working memory and short-term memory. Results showed that poor comprehenders were less able to use morphological analysis to interpret unfamiliar words while reading after controlling for vocabulary and morphological awareness. Vocabulary, morphological awareness and incidental word learning better discriminated the membership of poor comprehenders versus typical readers than working memory and short-term memory in both English-only and English language learners. The findings underscore the importance of lexical inferencing skills in reading comprehension regardless of language group. What is already known about this topic Poor comprehenders have weaknesses in oral language and working memory. Poor comprehenders have difficulties in some aspects of morphological awareness.