The effects of temperature and light on boll and shoot maturity and on the accumulation of fatty acids in developing seeds of flax (Linum usitatissimum L.) were determined in controlled environments. Palmitic and linoleic acids decreased but linolenic increased in percent as seed formation progressed. In the same period, oleic acid increased in percent in 1 variety and decreased in another. Increased temperatures hastened these changes and resulted in decreased iodine value of the oil at maturity.Calculated on a weight basis (mg per 1000 seeds), all 5 major fatty acids increased during seed formation. Increased temperatures initially accelerated the accumulation of all fatty acids, but the period of net fatty acid synthesis was eventually shortened in comparison with cooler temperatures. At 15 degrees and 20 degrees , linolenic acid accumulation closely paralleled the rate of boll maturation, measured by boll moisture content; at 30 degrees linolenic accumulation ceased before maturation could be detected.A photoperiod of 20 hours accelerated plant maturity resulting in decreased seed weight in comparison with a photoperiod of 16 hours. Eight hour photoperiod favored late blossoming and depressed seed weight, oil content, and fatty acid content. Weights of linoleic and linolenic acids were high in both the 16 and 20 hour photoperiods.Linolenic acid was reduced in percent and weight per 1000 seeds at light intensities of 1200 ft-c as compared with 2700 ft-c.