This paper presents a study of mobile data usage in South African townships. In contrast to previous studies, which have studied mobile data usage in developing regions (including South Africa), we focus our study on two townships in South Africa; the extremely resource-constrained nature of townships sheds light, for the first time, on how people in these communities use mobile data. We perform a mixed-methods study, combining quantitative network measurements of mobile app usage with qualitative survey data to gain insights about mobile data usage patterns and the underlying reasons for user behavior concerning mobile data usage. Due to the limited availability of public free Wi-Fi and despite the relatively high cost of mobile data, we find that a typical township user's median mobile data usage is significantly more than Wi-Fi usage. As expected, and consistent with observations of mobile data usage in parts of South Africa with better resources, users tend to favor using Wi-Fi for streaming video applications, such as YouTube. Interestingly, however, unlike users in less resource-constrained settings, township users also consume significant mobile data to update mobile applications, as opposed to relying on Wi-Fi networks for application updates. These behaviors suggest that network and mobile application designers must pay more attention to data usage patterns on cellular networks to provide mobile network architectures that provide more cost-effective mechanisms for tasks such as application update.