True potato seed of Atzimba × R128.6 (cross A) and Serrana x R128.6 (cross B) were tested to measure the effects of increasing berry development on seedling vigor. The seed were harvested at 6, 8, 10, and 12 weeks post-pollination and evaluated after about 6 months of storage. Influences of two pre-sowing treatments were also examined. The seed were soaked for 5 days in solutions of 1) KNO 3 + K 3PO 4 at -1.0 MPa (priming), and 2) gibberellic acid at 50 ppm (GA50); rinsed seed and untreated seed served as controls. After treatment, all lots were dried to uniform moisture and stored for 5 days before sowing in soil media under screenhouse conditions. Seed vigor levels were measured by 1) rate of emergence, 0-10 days after sowing (d.a.s.), 2) percentage of emergence, 10 d.a.s., and 3) seedling dry weight, 17 d.a.s. The stage of berry development at harvest was found to be an important factor influencing seed performance during early seedling growth. The association of vigor responses to increasing seed maturity was dependent on the treatment and the cross. For cross A, most treatment responses were linear, and also curvilinear, particularly the priming treatment. Cross B responses were similar, although less marked, and the control treatments were more affected by increasing seed maturity. Maximum vigor ratings in both crosses were estimated to be in seed harvested at about 11 weeks post-pollination. However, the results of a preliminary test suggested that the slightly lower vigor in 12-week seed was due to a higher level of seed dormancy. Cross A was more vigorous than B, and priming was the most effective pre-sowing treatment used and GA50 the least. © 1989 Springer.