This paper proposes an urban model to discuss the conversion of customary agricultural land to formal and informal residential land in developing country cities. Because customary land sales are insecure, migrant buyers face a risk of eviction, which affects land markets in non-trivial ways. Tenure risk and asymmetric information likely cause the extent of the city and its population to be too small. Empirical tests of the model for Bamako, Mali, confirm the existence of tenure insecurity and information asymmetry in the primary land market, but not in the secondary market, consistently with information revelation after initial sales by customary holders.