The interactions between the effects of salinity and temperature on germination of onion seeds, the effect of stage of salinization on onion yield, possible varietal differences in salt tolerance, and the effect of salinity on water and ion relationships of the plants were studied in laboratory and field experiments. Strong interaction was found between the effects of salinity and temperature on onion germination. At 12°C onion seeds germinated fully in vitro with water having an electrical conductivity (ec) of up to 30 dS/m. At 30°C germination was almost completely arrested by an ec of 20 dS/m. Lower temperature and higher salinity both reduced the overall rate of onion germination. In the field, continuous drip irrigation of onion with water having electrical conductivities of 1.2, 4.0, 6.0 and 8.0 dS/m resulted in severe die-off seedlings in the salinized plots during the first 90 days of growth and, consequently, in severely reduced yield of bulbs (50.08, 28.35, 4.12 and 0.44 kg/10 m2, respectively). Much of this effect could be averted by delaying salinization until the 2nd-leaf stage and even more by waiting until the 5th-leaf stage. Nine onion cultivars having broad genetic diversity were subjected to the same salinity treatments. No differences were found in the relative responses of the cultivars to salinity. Observations of plant physiological behavior under saline conditions were used to support explanations for the effects of brackish water irrigation on onion yield.