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Fragile X syndrome (FXS) and Down syndrome (DS) are the two most common genetic causes of intellectual disability (Dykens, Hodapp, & Finucane, 2000; Hagerman & Hagerman, 2002). In addition to [...], Purpose: To compare the phonological accuracy and speech intelligibility of boys with fragile X syndrome with autism spectrum disorder (FXS-ASD), fragile X syndrome only (FXS-O), Down syndrome (DS), and typically developing (TD) boys. Method: Participants were 32 boys with FXS-O (3-14 years), 31 with FXS-ASD (5-15 years), 34 with DS (4-16 years), and 45 TD boys of similar nonverbal mental age. We used connected speech samples to compute measures of phonological accuracy, phonological process occurrence, and intelligibility. Results: The boys with FXS, regardless of autism status, did not differ from TD boys on phonological accuracy and phonological process occurrence but produced fewer intelligible words than did TD boys. The boys with DS scored lower on measures of phonological accuracy and occurrence of phonological processes than all other groups and used fewer intelligible words than did TD boys. The boys with FXS and the boys with DS did not differ on measures of intelligibility. Conclusions: Boys with FXS, regardless of autism status, exhibited phonological characteristics similar to those of younger TD children but were less intelligible in connected speech. Boys with DS showed greater delays in all phonological measures than the boys with FXS and the TD boys. KEY WORDS: fragile X syndrome, Down syndrome, autism, phonology