Object markers alternate between a prefix and a suffix position in the Thetogovela dialect of Moro, an underdocumented Kordofanian language of Sudan. Although the alternation appears to depend on the morphosyntactic category of verb forms, we show that it actually follows from the tonal properties of these verb forms. Verb stems that are usually marked with a default, phonologically predictable leftmost high tone select prefix object markers. The high-toned prefix object marker appears inside the stem, and its high tone serves as the default tone of the stem, obviating the need for inserted high tone. Verb stems that impose other tone patterns, either all high or all low, select suffix object markers, a fact that we attribute to the incompatibility of high-toned prefix object markers with all-high and all-low tone patterns. The data are analyzed as a case of phonology conditioning prefix placement and overriding standard suffix position. Although such phonologically determined mobile affixes are rare in the world’s languages, the Moro case provides a new example of affix mobility based on a novel property, tone, and it underscores the need to incorporate such cases into the architecture of grammatical systems.