Only M.7 and V.1 as interstems induced vigorous (2.40 to 3.50 m³) scions along with P.18, MM.111, M.7 and Bud.118 as rootstocks alone. Using the yield efficiency (kg fruit/cm² trunk cross-sectional area) of ‘Ruby Jon’/MM.111 as standard reference, the treatments increased yield efficiency in 1998 up to 591%, 1127%, 927% and 600% in the vigorous, semidwarf, dwarf and ultradwarf groups respectively because of the relatively low yields of the nonprecocious MM.111. In 1999, despite the increased crop on MM.111 scions, the interstem/rootstock treatments still increased the yield efficiency up to 168%, 263%, 226% and 91% in the vigorous, semidwarf, dwarf and ultradwarf categories respectively. The ultradwarfs consistently induced fruit size about 25% smaller than the control in both years with the exception that G.65/CG.778 and G.65/G.30 produced fruit approximately 17% larger than the other ultradwarfs. Of the 10 dwarfs, three (G.30/Bud.118, Bud.9/G.30 and G.30) induced fruit size equal or greater than control in both years. In 1998 only G.30/P.18 induced large fruits in the semidwarfs. In 1999 all semidwarf treatments except Bud 9./MM.111 produced fruit size meeting or exceeding control. The vigorous treatments generally induced large fruit size. Based on yield efficiency and fruit size, 7 out of the 30 treatments offered substantial improvement over the MM.111 standard. The performance of the promising interstems and rootstocks must be further tested for adaptability to both our harsh continental climate and the wet/dry extremes in soil moisture before recommendation to growers.