Human populations in widely differing environments show seasonal differences in rates of birth and conception. Reasons advanced to explain this phenomenon include seasonality of temperature, rainfall, marriage, migration, workload, food supply, and ritual. Fertility data on a group of peasant cultivators in highland Lesotho show a bimodal pattern of birth seasonality that differs substantially from neighboring groups in South Africa. While climate, diet, workload, and marriage contribute indirectly to the overall pattern of conception and birth, only labor outmigration is significantly correlated. Because this population is dependent on regional and international economic networks, the factors that influence fertility are complex.