Metacognitive therapy (MCT) suggests that anxiety disorders are caused by the cognitive attentional syndrome (CAS), which is supported by dysfunctional metacognitive beliefs. Acceptance and commitment therapy (ACT) emphasizes the role of cognitive fusion and experiential avoidance in the development of these disorders. In this study, it was hypothesized that the ACT concepts of cognitive fusion and experiential avoidance would mediate the relationship between dysfunctional metacognitive beliefs and anxiety/stress symptoms. A longitudinal design was employed. One hundred and six nonclinical participants responded twice to an online survey with a lapse of nine months. Cognitive fusion at Time 2 mediated the effect of negative metacognitive beliefs at Time 1 on anxiety symptoms at T2, whereas both cognitive fusion and experiential avoidance mediated the effect on stress symptoms. Cognitive fusion mediated the effect of positive metacognitive beliefs only on stress symptoms. These results warrant further comparison of the MCT and ACT models.