An experiment dealing with production and quality of onion (Allium cepa L.) seeds was carried out at Caçador, SC, at an altitude of 950m, with cultivar EMPASC 351. The 3 x 3 factorial treatments were: 3 different lenghts of flower stems (0, 30 and 60cm) and 3 harvest times: early - with green umbels, normal - with 10% of umbels opened, and late - when aproximately 50% of umbels were opened. The highest yield and best quality of seeds were obtained the normal and late harvest times. The seeds were more vigorous when harvested at normal time, independently of lenght of flower stem. At early harvest time, the most vigorous seeds were produced by plants with 60cm flower stem.