Purpose: The authors examined (a) whether dose frequency of milieu communication teaching (MCT) affects children's canonical syllabic communication and (b) whether the relation between early canonical syllabic communication and later spoken vocabulary is mediated by parental linguistic mapping in children with intellectual disabilities (ID). Method: The authors drew on extant data from a recent differential treatment intensity study in which 63 toddlers with ID were randomly assigned to receive either five 1-hr MCT sessions per week (i.e., daily treatment) or one 1-hr MCT session per week (i.e., weekly treatment) for 9 months. Children's early canonical syllabic communication was measured after 3 months of treatment, and later spoken vocabulary was measured at posttreatment. Midpoint parental linguistic mapping was measured after 6 months of treatment. Results: A moderate-sized effect in favor of daily treatment was observed on canonical syllabic communication. The significant relation between canonical syllabic communication and spoken vocabulary was partially mediated by linguistic mapping. Conclusions: These results suggest that canonical syllabic communication may elicit parental linguistic mapping, which may in turn support spoken vocabulary development in children with ID. More frequent early intervention boosted canonical syllabic communication, which may jump-start this transactional language-learning mechanism. Implications for theory, research, and practice are discussed.