Applying Social Cognitive Career Theory to preservice teachers, a study examined to what extent dispositional optimism and a subjective sense of performance during an internship predict changes in career self-efficacy. The study first determined whether a sample of preservice teachers experienced positive changes in career self-efficacy after an internship. A sample of 66 preservice teachers at a large southern university completed a measure of career self-efficacy and the LOT-R (Life Orientation Test-Revised), which measured dispositional optimism prior to beginning their internship. After a three-month internship, the subjects again completed measures of career self-efficacy and subjective performance ratings. Results indicate that the best predictors of increasing career self-efficacy during an internship for preservice teachers were the following: (1) level of preexisting career self-efficacy; and (2) subjective beliefs about their performance during the internship. These factors had a greater effect than the personal trait of dispositional optimism. The findings suggest internships may be a learning experience that successfully leads to increases in career self-efficacy. In addition to showing the effectiveness of internships in increasing career self-efficacy, the study shows the importance of providing self-relevant performance feedback during an internship. (Appendixes include the survey instruments and 34 references.) (YLB)