The C(2)H(2) zinc finger proteins form one of the largest families of transcriptional regulators in eukaryotes. We identified a Phytophthora sojae C(2)H(2) zinc finger (PsCZF1), that is highly conserved in sequenced oomycete pathogens. In transformants of P. sojae containing the PsCZF1 promoter fused to the beta-glucuronidase (GUS) reporter gene, GUS activity was highly induced in the P. sojae oospore stage and upregulated after infection. To elucidate the function of PsCZF1, its expression was silenced by introducing anti-sense constructs into P sojae. PsCZF1-silenced transformants did not exhibit altered cell size or morphology of sporangia and hyphae; however, hyphal growth rate was reduced by around 50% in the mutants. PsCZF1-deficient mutants were also impaired in production of oospores, swimming zoospores and germinating cysts, indicating that the gene is involved in various stages of the life cycle. Furthermore, we found that PsCZF1-deficient mutants lost virulence on host soybean cultivars. Our results suggest that this oomycete-specific C(2)H(2)-type zinc finger protein plays an important role in growth, development, and pathogenesis; therefore, PsCZF1 might be an attractive oomycete-specific target for chemical fungicide screening.