This study investigated the protective effects of various phytohormones and plant activators on soyabean resistance to Phytophthora sojae, the cause of root and stem rot. It was found that application of benzothiadiazole (BTH, an activator of salicylic acid (SA) signalling) and 1-aminocyclopropane-1-carboxylic acid (ACC, a precursor of ethylene (ET) biosynthesis) markedly induced resistance to P. sojae. By contrast, gibberellin (GA) and abscisic acid (ABA) rendered soyabean seedlings more susceptible to P. sojae. Simultaneous application of ABA with BTH or ACC suppressed the protective effects of BTH and ACC, indicating that ABA acts antagonistically on the SA- and ET-signalling pathways. Neither BTH nor ACC directly inhibited growth of P. sojae. The protective effect of ACC was diminished by co-treatment with its analogue α-aminoisobutyric acid, suggesting that ET biosynthesis is required for ACC-induced soyabean resistance. Expression analysis of ET- and SA-responsive genes demonstrated the activation of ET- and SA-signalling pathways during P. sojae infection. Furthermore, ACC treatments augmented the expression of ET-responsive and pathogenesis-related genes. Taken together, the results indicate that ACC (ET)-induced soyabean resistance to P. sojae relies on transcriptional augmentation of defence-related genes.