Zoospores are a critical component of the disease cycles of most oomycete pathogens. To better understand this stage, genes induced during zoosporogenesis were identified from Phytophthora infestans, the potato late blight pathogen. Using cDNA arrays representing 2,600 genes expressed during zoosporogenesis, 69 genes showing >fourfold increases in mRNA levels were identified, of which 22 exhibited >100-fold induction. Included were putative protein kinases, transcription factors, ion channels, and other regulators. The expression of 15 genes was characterized in detail using zoosporogenesis time courses, other developmental stages, different temperature regimes, and tissue treated with signaling inhibitors. The latter were of interest because zoosporogenesis is known to be cold induced and inhibited by calcium channel blockers such as verapamil; moreover, in this study, inhibitors of phospholipase C (U-73122) and inositol trisphosphate receptor-gated calcium channels (2-aminoethoxydiphenyl borate) also were shown to block zoosporogenesis. The results indicated that the cytoplasmic and transcriptional changes occurring during zoosporogenesis are regulated by several pathways. For example, verapamil inhibited zoosporogenesis but not the up-regulation of most genes; the induction of some genes required while others were independent of calcium or phospholipid signaling; and, although most genes were induced in sporangia at 10°C but not 24°C, one was induced at both temperatures.