This paper examines the acquisition of the grammatical tone system of Sesotho, a southern Bantu language where tone sandhi is rich, and where surface and underlying representations are often quite distinct. Results of the longitudinal case study show that rule-assigned tone on subject markers is generally marked appropriately by age two. In contrast, underlying tonal representations on verb roots are learned gradually over time, showing an early Default High tone pattern. The study also finds that, while some tone sandhi rules are in the process of being acquired between 2; 6 and 3;o, problems in the mapping between tonal representations and segments persist. The paper raises methodological and theoretical issues not only for the acquisition of tonal systems, but for the acquisition of phonology in general.