This study investigated two widely-used early literacy skill's indicators in reflecting growth in first-grade language achievement skills. It compared two curriculum-based assessments of letter knowledge: Letter Naming Fluency (LNF) and Letter Sound Fluency (LSF) in the Arabic language. A sample of 125 first-grade students, 100 average readers and 25 with reading difficulties, was recruited from two public primary schools in Jordan. These students were administered both assessments 18 times, one each week, during the second half of the first grade. Students' progress for both measures was followed. Although students' LNF progress was higher than LSF, still LSF had a higher correlation with Arabic language achievement than LNF at the end of the first grade. Furthermore, students who were struggling with reading scored significantly lower on Arabic CBM LNF and LSF probes than their peers without disabilities. Implications for Arabic language learning and assessment are discussed.