Given the visual-gestural nature of ASL it is reasonable to assume that visualization abilities may be one predictor of aptitude for learning ASL. This study tested a hypothesis that visualization abilities are a foundational aptitude for learning a signed language and that measurements of these skills will increase as students progress from beginning ASL students to advanced language learners and, ultimately to credentialed interpreters. Participants in this study consisted of 90 beginning and 66 advanced ASL students in five interpreter education programs in four southern states along with 68 credentialed interpreters. Students and interpreters were administered the "Vividness of Visual Imagery (VVIQ)" self-report questionnaire and the objective "Mental Rotations Test, Version A (MRT-A)". All ASL students and their instructors were asked to rate students' sign language competency on the "Sign Communication Proficiency Interview Rating Scale". All participants completed demographic questions regarding their age, gender, ethnicity, parental hearing status, number of years using ASL, number of years working with deaf professionals who use ASL, and their interpreting credential(s). Students and their instructors rated students' sign communication proficiency similarly. Beginning ASL students were rated significantly lower than the advanced ASL students by both instructors' rating and students' self-rating. No significant relationships were reported: a) among beginning and advanced students and credentialed interpreters with respect to either the VVIQ or the MRT-A, and b) among the students' VVIQ and MRT-A scores and instructors' ratings on the SCPI. There was suggestive evidence of an increase in mean VVIQ scores from beginning ASL students to advanced ASL students to credentialed interpreters, but not to the level of significance. When advanced ASL students and lower level state credentialed interpreters were removed from analyses, a significant difference in visual vividness was reported. Nationally certified interpreters scored significantly higher than beginning ASL students on the VVIQ, but not the MRT-A. [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.]