Purpose: This study compared and characterized the tense and agreement productivity of boys with fragile X syndrome (FXS), children with developmental language disorder (DLD), and children with typical development (TD) matched on mean length of utterance. Method: Twenty-two boys with FXS (M[subscript age] = 12.22 years), 19 children with DLD (M[subscript age]= 4.81 years), and 20 children with TD (M[subscript age] = 3.23 years) produced language samples that were coded for their productive use of five tense markers (i.e., third-person singular, past tense "-ed," copula "BE," auxiliary "BE," and auxiliary "DO") using the tense and agreement productivity score. Children also completed norm-referenced cognitive and linguistic assessments. Results: Children with DLD generally used tense and agreement markers less productively than children with TD, particularly third-person singular and auxiliary "BE." However, boys with FXS demonstrated a more complicated pattern of productivity, where they were similar to children with DLD and TD, depending on the tense marker examined. Results revealed that children with DLD and TD showed a specific developmental sequence of the individual tense markers that aligns with patterns documented by previous studies, whereas boys with FXS demonstrated a more even profile of productivity. Conclusions: These findings help to further clarify areas of overlap and discrepancy in tense and agreement productivity among boys with FXS and children with DLD. Additional clinical implications of these results are discussed.