According to sociolinguistics, languages can be called promary languages and secondary languages. The former is the one more often used by somebody in everyday life,even through ot my not be their first language. The secondary language is therefore is the one more or less used by somebody. for example, for reason that it is less important as a means of every day communication. for many Indonesians today, particularly in large cities,Indonesian is their primary language, while their regional languages is the secondary language. This paper attempts to discuss the diglosic shift involving Indonesian and the mother tongue (read:regional language). What is meant by diglosia is the language situation where there are two languages (or two language registers) which have divided functions: one has a High function (H) and the other a Low function (L). Also discussed here is the implication of such diglosic shift for the maintenance of the mother tongue. The term regional language is used intentionally in the discussion, not mother tongue for the reason that Indonesian for many Indonesians has become their first language or mother rongue, and it is argued that Indonesian as a mother tongue is not under any threat of shift.