Technical Abstract: Conyza canadensis L. Cronq. is a common pest in vineyards of the San Joaquin Valley (SJV) of California, USA. Interest in controlling this weed has increased with the recent discovery of a glyphosate-resistant (GR) biotype which is more vigorous than a glyphosate-susceptible (GS) biotype. However, the impact that either biotype may have on grapevine (Vitis vinifera L.) growth has not been assessed. Therefore, two experiments were conducted in a glasshouse to characterize the competitiveness of GR and GS C. canadensis biotypes with young grapevines. ‘Syrah’ grapevines grafted to Freedom rootstocks were planted in 8 L plastic pots, alone, or with a single GR or GS weed. Additional GR and GS weeds were also planted separately in individual pots, and all plants were grown for 14 and 16 weeks in 2006 and 2007, respectively. Grapevines grown with either biotype of the weed produced fewer leaves and amassed approximately 20% less dry matter (D 24 M) than vines grown alone. The GR biotype of the weed reduced grapevine stem DM and length by 30%, but the GS biotype did not. The GR biotype accumulated more than twice the DM as the GS biotype, whether in competition with grapevine or not. Grapevines reduced the total leaf number of C. canadensis by almost 50% and plant DM of both the biotypes (50% in GR and 75% in GS biotype), but not stem height. These findings indicate that competition from C. canadensis can substantially reduce the growth of young grapevines, and that the GR biotype may be more competitive than the GS biotype.