Background: New emerging online technologies have made dynamic changes in the structure of the online environment possible to address varying needs of diverse learners. Purpose: The purpose of this study was to identify the changes that occurred in online instructional design and strategies in mathematics education courses taught over a course of six years by the same instructor. The research questions were: What are the changes / revisions that occur in teaching online graduate courses over the course of six years and what are the reasons for these changes as explained by the instructor? Methods: The research method used was a qualitative case study utilizing recorded and transcribed instructor interviews, observation of synchronous online classes and archival data in the form of course syllabi and PowerPoint slides used in the synchronous online class sessions. The study participant, a veteran mathematics educator taught online mathematics education courses for six years in addition to being involved in the design, implementation and evaluation of an online graduate program. Carspecken's critical qualitative research method was used for interview protocols and data analysis. Results: The findings of the study revealed changes that occurred in the online course design structure, synchronous online class patterns, activity structures using synchronous and asynchronous portions of the online courses, instructional and knowledge sharing strategies, and choice of technological tools and related activities. The need to ensure students understanding and engagement, establish collaborative learning communities, provide required feedback and develop technological and research skills were some of the factors that contributed to the design and redesign of online course and activity structures during the six years of online instruction. Conclusion: A continual cycle of design and redesign occurred in the online course structure, choice of activities fostering collaborative knowledge construction and technology use. The knowledge of changes occurring in the online course design and instructional decisions that lend themselves to those changes can provide opportunities that harness the instructional design potential of online instructors as well as offer a guideline to better understand the process of transforming course content online. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]