Interlanguage development of foreign/second language learners has taken great interest in the Second Language Acquisition (SLA) domain. Whether these learners have access to the principles and parameters; that is, whether Universal Grammar (UG) is available to them have been a controversial issue for researchers. One of the parameters is the pro-drop parameter which allows subject pronouns to be omitted in certain languages such as Japanese and Turkish. However, studies on Turkish, a [+ prodrop] language, are rare. This study aims at investigating whether native Turkish learners reset their L1 pro-drop parameter value when they learn a [- prodrop] language such as English and whether these learners have access to UG in the initial, intermediary and advanced levels of their interlanguage development. The findings of the study has clarified how parameter setting takes place not only in the initial state but also in the later stages of interlanguage development.