Dynamic assessment (DA) is based on Vygotsky's (1978) sociocultural theory and his Zone of Proximal Development (ZPD). ZPD is the range of abilities bordered by the learner's assisted and independent performances. Previous studies showed promising results for DA in tutoring settings. However, they did not use proficiency-based rubrics to measure students' progress and did not mention the method of using DA practically in classrooms. The literature showed that task-based language instruction (TBLI) is effective in adult classrooms. This study combined DA with TBLI to answer four questions. What is the change in the structural control of Arabic speaking based on DA/TBLI instruction? How do Oral Proficiency Interview (OPI) without DA assistance and OPI with DA assistance compare relative to the evaluation of Arabic speaking? How do the experiences and perceptions of DA/TBLI instruction compare between teacher-researcher and OPI testers? What are the student perceptions of the DA process? The study was conducted in three phases to answer its questions: pre-DA, DA, and post-DA. In the pre-DA phase, 12 volunteers from the Defense Language Institute Foreign Language Center went through unofficial Oral Proficiency Interviews (OPI), intellectual style survey, biographical background questionnaire, and interventionist-DA interviews. During the DA phase, the teacher-researcher used DA/TBLI instruction and Interagency Language Roundtable-based (ILR) rubrics to promote learning and to diagnose students' needs daily. These lessons were observed by certified OPI testers. In the post-DA phase, the six selected participants were reevaluated by OPIs and interventionist-DA interviews. Students and observers were interviewed, but only students responded to a survey. The results of comparing the different evaluations conducted in both the pre- and post-DA phase showed that the structural control of Arabic improved for all participants. There is a parallel coefficient of 1.0 between the OPI with and without DA assistance for evaluating the participants' speaking proficiency. DA/TBLI instruction was practical and successful in making a difference for the participants' learning process. It reflected the success of the ILR-based rubrics in diagnosing accurately the students' inabilities whether in the interventionist-DA interviews or in the daily interactionist DA. The OPI without DA assistance cannot provide accurate diagnostic feedback in details. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]