The effect of nearly constant water potential on germination of citrus, sunflower, and lettuce seeds was studied. Water potential equilibration was achieved by placing soil above a cellulose acetate membrane which was in contact with a solution of polyethylene glycol-6000. Selection of solute potentials in the solution resulted in controlled water potentials in the soil over a range of 0 to −14.9 bars for citrus and 0 to −8.0 bars for sunflower and lettuce. The water stress experienced by germinating seeds in this system is largely the result of a matric effect rather than a solute effect.Citrus seeds germinated at water potentials as low as −4.7 bars and lettuce at −4.1 bars, but sunflower germinated at −8.0 bars. Sunflower germinated as rapidly at −4.1 bars as lettuce at −2.3 bars, both reaching 50% germination at about 8 days. Citrus germinated much more slowly, requiring 26 days at 0 bars.