This preliminary study investigated the effects of co-treatment by an adapted physical educator and speech-language pathologist on basic concept vocabulary learning in preschool children with Down syndrome as compared with the effects of treatment by either professional alone. In a Repeated Acquisition Design/Adapted Alternating Treatment study, 10 preschool children with Down syndrome were taught five basic concept words each week in a co-treatment intervention, in adapted physical education alone, or in speech-language therapy alone across 9 weeks. Each week, participants participated in a pre- and posttest. Four children learned the most words in the co-treatment condition and the remaining six children did not learn different numbers of words across conditions. Individual characteristics of children in the study were evaluated to determine possible factors that differentiate learning patterns among children. This preliminary study provides evidence that co-treatment may be effective for vocabulary teaching for some children with Down syndrome, particularly those with relatively high nonverbal intelligence.