Dinka, a major Western Nilotic language spoken in the Sudan, is to a large extent a monosyllabic language. Nevertheless, it has a complex morphology. Thus a significant part of its morphology is non-affixal, being manifested by way of morphophonological alternations in the root. Such alternations involve one or more of the following parameters: vowel quality, vowel length, voice quality, tone and final consonant. While alternations in vowel length, voice quality and tone are treated in Andersen (in press), the present article deals with vowel quality alter-nation. The dialect of Dinka treated here is Agar, more specifically the variety of Agar spoken by people from the area of Pacong, a village about 20 kilometres south-east of Rumbek in Southern Sudan. My principal informants for the present study were Isaac Maker, David Daniel Marial and Peter Gum Panther.