The study examines the sex imbalance at the marriageable ages according to ethnic group and religion, utilizing data obtained from the complete tabulations of the 1971 Census of Population in Sri Lanka. The active enforcement of Indian repatriation, commencing in 1971, played a major role in increasing net migration rates to significant levels in 1971, 1972, 1973, 1974 and 1975 and thereby reducing the island's growth rate to 1·45% and 1·71% in 1974 and 1975. These out-movements were further increased by the exodus of Sri Lanka citizens which began in 1972 and continued in the years 1973–75, presumably dominated by males aged 20–44 years. This emigration of Sri Lanka citizens is expected to continue and will aggravate further the difficulties of contracting a marriage within each ethnic and religious group. Possible demographic consequences are suggested.