Research on the acquisition of grammatical gender has shown that for many languages, children gain an early command of gender. However, often in these languages gender marking is quite overt and provides a clear one-to-one correspondence between a marker and the gender encoded. In Welsh, gender marking is more complex. Gender is marked by mutations, a set of morphophonological changes that affect the initial consonants of words, and the mapping between mutation and gender is quite opaque. Two mutation types are used in part to mark feminine gender: both feminine nouns modified by the definite article and adjectives following feminine nouns undergo Soft Mutation, and the feminine gender of the possessive adjective ei is marked by Aspirate Mutation on the modified noun. The four studied in this thesis examined children's productive command of gender as expressed in the mutation of nouns modified by the definite article, of adjectives modifying nouns, and of nouns modified by the homonymic feminine and masculine possessive adjective. Mutation in non-gendered contexts was also examined. Subjects were 4- to 9 1I2-year-old children from North Wales. First, a seminaturalistic study was conducted to obtain knowledge about children's ability with gender marking. A Cloze procedure was also used to elicit children's production of masculine and feminine forms, both real words and nonsense forms, in a variety of linguistic contexts. Some of these contexts provided cues to gender status, some did not. The data obtained indicated that the acquisition of the Welsh gender system is a drawn-out process, and children have not mastered the system even by 9 112years of age. In addition, children become proficient in marking feminine nouns modified by the definite article and adjectives modifying feminine nouns before they do so on nouns modified by feminine ei. Results suggest that when a language has a complex gender system that is marked by opaque morpho-phonological processes the course of development is protracted and variable.