“Diglossia is a natural phenomenon in any language: If there is any difference between diglossia in Arabic and in other world languages like English or French, it is a quantitative difference” (Zughoul, 1980:32). This paper discusses Arabic diglossia in contrast with the diglossic situation in English with special reference to the quantitative difference. A comprehensive definition of the term “diglossia” is also provided. Besides, the paper focuses on issues related to the linguistic implications of diglossia including phonological, lexical, and syntactic differences between the standard and colloquial varieties of English and Arabic. Finally, a special section in the paper discusses whether or not diglossia is a stable language situation and how it can be a real obstacle in the communicative function in a language which is highly diglossic.