Migration was a crucial component of the spatially uneven formation of labour markets and export-oriented economies in colonial Africa. Much of this mobility was initiated by migrants themselves rather than by colonial authorities. Building on analytical concepts from economic history and migration theory, this study explains the changing composition and magnitude of one such uncontrolled migration flow, from Ruanda-Urundi to Buganda. Migrants’ mobility choices – when to migrate, for how long, and with whom – proved highly responsive to shifting economic opportunity structures on the sending and receiving ends. Initially, large differences in terms of land and labour endowments, socio-economic structures, and colonial interventions, combined with substantial scope for price arbitrage, created large spatial inequalities of opportunity and strong incentives for circular male labour migration. Over time, however, migration contracted as opportunities in Ruanda-Urundi and Uganda converged, not in the least as a result of large-scale mobility itself.