To analyze and evaluate textbooks, researchers have either proposed scales and checklists to be filled by teachers and learners or conducted qualitative investigations of the match between SLA theories and textbook activities. This study, however, employs the microstructural approach of schema theory to scrutinize the reading passages of “Mosaic 1 Reading”. To this end, 17 passages of the textbook were randomly chosen and their constituting words were explored as semantic, syntactic, and parasyntactic schemata. The passages were also analyzed in terms of their readability indices. The results showed that they consist of 3722 schema types, 2979 (80%) of which are semantic in nature. Although the textbook aims at “academic success” at English language “proficiency levels”, it provides no objective definition of what they stand for. In terms of readability, however, the passages vary in difficulty from grade three in primary school to college level. Further, the textbook is discussed in terms of its constituting schemata and suitability to the Iranian context and suggestions are made for future research. The findings of this study have important implications for language teaching, testing and materials development. They show that language proficiency must be defined in terms of schema types and the bulk of class time must be spent on teaching semantic schemata rather than syntactic and parasyntactic ones. Similarly, for testing the reading comprehension of these passages, the number and type of test items must be based on the percentage of semantic and syntactic schema types and subjective criteria such as teachers’ intuition or experience must be avoided both in teaching and testing the comprehension of passages.
