Ternary or three-level vowel length is typologically rare, and supporting evidence is limited. This paper presents the results of an investigation into the hypothesised case of this configuration in Shilluk. We first describe the role of vowel length in Shilluk phonology and morphology, and then report on an acoustic study in which minimal sets for vowel length (short, long, overlong) are measured for vowel duration, coda duration, vowel quality and fundamental frequency. Short, long and overlong vowels differ significantly and substantially in terms of vowel duration: 96% of the items can be classified successfully for vowel length on the basis of this measurement alone. Of the other measurements, only vowel quality is significant, and this effect is considerably smaller. The mean values for vowel duration – 68, 111 and 150 ms for short, long and overlong vowels respectively – are similar to those reported for ternary vowel length in Dinka.