Hybrid rootstocks from a United States Department of Agriculture citrus breeding program were compared for their resistance to phytophthora collar rot (caused by Phytophthora citrophthora) and citrus tristeza virus (CTV), with 6 rootstocks used commercially in Australia. Rootstocks most resistant to collar rot were clone 22 of Poncirus trifoliara, F1 Cleopatra mandarin x English P. trifoliara, and Benton citrange. Mean lesion lengths for F1 Sunki mandarin X Mars P. trifoliata, Troyer citrange, (Rangpur lime x Swingle P. trifoliata) open pollinated, and F, Cleopatra mandarin X Christiansen P. trifoliata were not significantly different from clone 22 of P. trifoliata. Some P. trijoliata hybrids were more susceptible to collar rot than sweet orange, Cleopatra mandarin, or rough lemon. High titres of CTV in ELISA tests were evident in some hybrids, but only 1 F1 Rangpur lime X Carrizo citrange developed stem pitting symptoms. CTV titres in late spring (November) were higher in bark than in leaves.