Native Arabic speakers read in a language variety that is different from the one they use for everyday speech. The aim of the present study was: (1) to examine Spoken Arabic (SpA) and Standard Arabic (StA) voweled and unvoweled word reading among native-speaking sixth graders with developmental dyslexia; and (2) to determine whether SpA reading ability among children with dyslexia predicts StA reading fluency in the two orthographies: voweled and unvoweled. A comparison was made to three age groups of typically developing children: a group matched by chronological age, a group of children who are two years younger, and a group of children who are 4 years younger. Findings show that diglossia has a strong impact on reading ability in dyslexic children. Moreover, vowelization plays a pivotal role in the reading ability of Arabic-speaking children with dyslexia in both SpA and StA. This role is evident in the different performance patterns of dyslexic participants as compared with controls on word-reading accuracy and fluency for SpA versus StA. Finally, StA word-reading fluency appears to depend on and to be reliably and powerfully predicted by SpA word-level reading ability. These results underscore the role of diglossia and vowelization in the manifestation of dyslexia in Arabic-speaking children.