Although much has been discovered about relations between self-efficacy and academic achievement, questions remain about links between achievement, the structure of learning tasks, and changes in students' self-efficacy as students engage with a single, complex authentic task. Students' self-efficacy for learning (SEL) and for performance (SEP) was tracked as they worked on well- and ill-structured tasks during their regular class. Students reported higher SEL and SEP for a well-structured task. Moderate achievers reported significantly more difficulty with the ill-structured task. SEP was higher and more stable than SEL, especially in early phases of both tasks. After accounting for overall academic achievement, self-efficacy was a negligible predictor of achievement. Students may perceive various features of each task's structure as difficult. Implications concerning relations among self-efficacy, task structure, and achievement are discussed.