Six onion (Allium cepa L.), cultivar Pira Ouro, seed lots were studied to determine the efficiency of vigor tests in evaluating the physiological quality of the seeds correlating the results with field emergence and production of vigorous seedlings. The trial was conducted at the seed analyses laboratory and experimental field of the Escola Superior de Agricultura ?Luiz de Queiroz?, University of São Paulo, Piracicaba campus, São Paulo state, Brazil. The evaluations were performed by using the foIlowing tests: standard germination, first count, cold test, accelerated aging, electrical conductivity, and field emergence. Seedling quality was determined at the transplanting time, considering vigorous the seedlings presenting lenght equal or longer than 20cm and diameter equal or thicker than 5mm, as welI as by the dry matter of vigorous seedlings. The cold and accelerated aging tests showed better correlation with field emergence and vigorous seedlings for transplanting than the other tests, also being useful in identifying seed lots with different levels of vigor. The standard germination test overestimated the performance of vigorous seedlings for transplanting.