This study examined two common accounts of heritage speakers' nonnative attainment in their heritage/first language (L1), one attributing it to the influence of the second language (L2) and another to insufficient L1 input. Three groups of children who were heritage speakers of Arabic and who varied in their age of L2 exposure and type and amount of L1 input (n = 31) were compared to monolingual Arabic-speaking children (n = 12).The participants completed three oral tasks targeting subject-verb agreement, plural morphology, and relative clauses. The findings revealed significant differences among the groups, except between the monolingual group and the group of heritage speakers that had not been exposed to English at the time of the study. Although both L2 exposure and L1 input correlated positively with the children's overall accuracy, age of L2 exposure was the only significant predictor of accuracy. The disparity in the groups' performance was also explained by the properties of the linguistic forms under study.