According to Cognitive Load Theory, learning material should be designed in a way to decrease unnecessary demands on working memory (WM). However, recent research has shown that additional demands on WM caused by less legible texts lead to better learning outcomes. This so-called disfluency effect can be assumed as a metacognitive regulation process during which learners assign their cognitive resources depending on the perceived difficulty of a cognitive task. Increasing the perceived difficulty associated with a cognitive task stimulates deeper processing and a more analytic and elaborative reasoning. Yet there are studies which could not replicate the disfluency effect indicating that disfluency might be beneficial only for learners with particular learner characteristics. Additional demands on working memory caused by disfluent texts are possibly just usable by learners with a high working memory capacity. Therefore the present study investigated the aptitude-treatment-interaction between working memory capacity and disfluency. Learning outcomes were measured by means of a retention, a comprehension, and a transfer test. Moreover, the three types of cognitive load (intrinsic, extraneous, and germane) were assessed. The results revealed significant aptitude-treatment-interaction effects with respect to retention and comprehension. Working memory capacity had a significant influence only in the disfluency condition: The higher the working memory capacity, the better the retention and comprehension performance in the disfluency condition. No effects were found with respect to transfer or cognitive load. Thus, the role of metacognitive regulation and its possible effects on cognitive load need further investigation.