Phytophthora sojae and Arabidopsis thaliana were used in this study as a nonhost plant-oomycete interaction system to investigate the genetic basis of nonhost resistance against oomycete pathogens in plants. A collection of more than 40 000 T₃ A. thaliana T-DNA mutant plants representing 12 000 independent insertion lines were screened by inoculating detached leaves with P. sojae zoospores, and the susceptible mutant was re-confirmed by pathogen inoculation and cytological characterization. A number of P. sojae-susceptible A. thaliana mutants were successfully obtained, and one of them, mutant 581-51, was shown to be stably susceptible to P. sojae infection. Water-soaked lesions formed on the detached leaves within 3 days, as did oospores and sporangia 4–5 days after inoculation with P. sojae zoospores. Cytological characterization revealed the formation of haustoria-like structures. Southern analysis showed the presence of four T-DNA insertion events in the mutant. Genetic analysis indicated that the susceptibility to infection by the nonhost pathogen P. sojae in the mutant 581-51 was likely controlled by a single recessive gene.