In order to produce onion seed in Sri Lanka, two trials were conducted by using cv. 'Poona Red'. In the first trial, the temperature required for flower bud differentiation and the duration of the period of low temperature treatment were determined. In the second trial, the actual seed production of onion based on the first trial was analysed. Exposure of mother bulbs to the highland conditions for one month (temperature min. : 12.5°C, max. : l8.8°C) was effective for flower bud differentiation. The use of a house with a plastic film roof was also effective in protecting the plants from diseases and the flowers from rain though devernalization was observed to some extent.