This article presents a case of allotony based on the phonation of the vowel in Nuer, a Western Nilotic language; the falling tone is found only on modal vowels in this language, while the high level tone is found only on breathy vowels. We describe the phenomenon and present evidence suggesting that it may be due to the neutralization of two separate tonal contours, H and HL, conditioned by the phonation of the vowel. We place this phenomenon within the known typology of phonation-tone interaction and advance a proposal as to the phonetic factors behind its development.