In monolingual development, the acquisition of differential object marking (DOM) is completed by three years of age (Rodríguez- Mondoñedo, 2008). However, among bilingual speakers, the development and use of the marker at a young age is less predictable. Spanish marks animate and specific direct objects with the preposition-a; English in contrast does not. Based on previous studies documenting transfer in areas where Spanish and English differ, it was predicted that bilingual children would experience difficulties with the use of the preposition both in matrix and left dislocated sentences (CLLD) (Montrul, 2004, Montrul & Bowles, 2009). This study tested 14 simultaneous Spanish-English bilingual children divided into a younger (6;04-7;09) and an older group (8;06-10;10) as part of a larger study. Six parents participated as a baseline to control for the possible acquisition of a variety in which the personal-a is not used (Rothman, 2007). The use of DOM was elicited in animate and inanimate specific contexts through a question and answer task for matrix sentences (Thornton, 1990) and a sentence completion task for CLLD sentences (e.g., Cuza, Pérez-Leroux & Sánchez, 2012). The results show that older children have some knowledge of the use of the marker in matrix sentences, but the younger group showed more difficulties. In CLLD sentences both groups had significant difficulties in the production of personal-a. In contexts where DOM is not required, both groups showed ceiling performance. There were significant differences between the group of children and the parents, who, in general, performed target-like. This supports the claim that bilingual children\u27s difficulties stem from transfer and are not the representation of a contact variety. Patterns of language used at home and input factors also account for the differences found