Purpose: The current study investigated finiteness marking (e.g., he walk "s", he walk "ed") in boys with fragile X syndrome (FXS); the boys were grouped based on receptive vocabulary (i.e., borderline, impaired). Method: Twenty-one boys with the full mutation of fragile X, between the ages of 8 and 16 years participated. The boys completed probes from the Test of Early Grammatical Impairment (TEGI; Rice & Wexler, 2001), a language sample, a nonverbal IQ test (Leiter-R; Roid & Miller, 1997), a receptive vocabulary test (the Pearson Picture Vocabulary Test-Fourth Edition [PPVT-IV]; Dunn & Dunn, 2007), and a measure of autistic symptoms (the Childhood Autism Rating Scale [CARS]; Schopler, Reichler, & Renner, 2002). Results: There were group differences for finiteness responses on the 3rd person singular probe; the group with impaired vocabulary omitted markers with greater frequency compared to the borderline vocabulary group. There were not significant differences on the past tense probe, with both groups performing lower than expectations based on receptive vocabulary ability. Nonverbal IQ was not correlated with the measures of finiteness marking. Conclusion: Boys with FXS demonstrate delays in finiteness marking, in particular, on past tense verbs. Boys with FXS show a unique profile, unlike children with SLI, in which their use of tense markers may exceed expectations benchmarked to clause length.