Zoospores are critical in the disease cycle of Phytophthora infestans, a member of the oomycete group of fungus-like microbes and the cause of potato late blight. A protein kinase induced during zoosporogenesis, Pipkz1, was shown to interact in the yeast two-hybrid system with a putative bZIP transcription factor. This interaction was confirmed in vitro using a pull-down assay. The transcription factor gene, Pibzp1, was single copy and expressed in all tissues. Transformants of P. infestans stably silenced for Pibzp1 were generated using plasmids expressing its coding region in sense or antisense orientations. A protoplast transformation method induced silencing more efficiently than transformation by an electroporation scheme. Wild-type and silenced strains exhibited no differences in hyphal growth or morphology, mating, sporangia production or zoospore release. However, zoospores from the mutants spun in tight circles, instead of exhibiting the normal pattern of straight swimming punctuated by turns. Zoospore encystment was unaffected by silencing, but cysts germinated more efficiently than controls. Germinated cysts from the mutants failed to develop appressoria and were unable to infect plants; however, they could colonize wounded tissue. These phenotypes indicate that Pibzp1 is a key regulator of several stages of the zoospore-mediated infection pathway.