Professional development is viewed as a lifelong learning process for educators to improve student instruction and professional skills (Darling-Hammond et al., 2017). Online professional development for teachers of students who are visually impaired (TVIs) provides opportunities for continuing education, including for earning of recertification hours. Since visual impairment is a relatively low-incidence condition in young people, distance education for TVIs is especially attractive. With the rapid growth of online professional development, there is a need to assess the efficacy of synchronous and asynchronous options (Rowe, 2019). The purpose of this quasi-experimental study was to compare two types of abacus professional development provided to TVIs--online synchronous and online asynchronous--to determine if a significant difference existed for growth of abacus knowledge, participant engagement, and participant satisfaction. Moore's transactional distance theory (1973) provided a conceptual framework for the study. The results of the analyses found that participants (N = 93) in both conditions had learning gains. The repeated-measures analysis of variance (ANOVA) suggested the participants in the asynchronous condition had greater gains in abacus knowledge, but the proximity of the p-value (p=0.051) to the customary threshold of 0.05 makes it difficult to use the p-value to draw conclusions regarding the statistical significance of this difference. Participants reported approximately the same levels of engagement or satisfaction in both conditions. This study adds to the very limited literature on differences in educational outcomes for online synchronous and asynchronous professional development for TVIs. The fact that online asynchronous professional development appears to have impacted learning growth more than online synchronous professional development was intriguing. This study suggests that online learning environments can be engaging and satisfying whether delivered synchronously or asynchronously. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]