AbstractThis paper presents an experimental study of the acquisition of rootwh-questions in French, in whichwh-movement is optional. Thirty-twowh-questions were elicited from 33 French-speaking children and 22 adults to allow for a quantitative and qualitative comparison with respect to the various word orders. A comparison reveals the following findings: 1° Children produce morewh-in-situ structures; 2° Children produce no subject-verb inversion structures while adults use them often; 3° The choice of question-structure is related to thewh-word that is used, for all subjects; 4° Children are aware of adults’ preferences but do not imitate them; 5° They acquire the various structures for eachwh-element separately; 6° The acquisition of optional rules reflects an interaction of external factors (the input) and internal factors (economy).