Significance We describe here the discovery that assemblies of O -fucosylated proteins localize to the nuclear membrane of Toxoplasma gondii , particularly in proximity to the nuclear pore complexes (NPCs). O -fucose is added to Ser and Thr residues found in some of the Phe-Gly (FG) domain-containing proteins that characterize the NPC channel as well as in Ser-rich sequences in many proteins predicted to have roles in transcription, mRNA processing, and cell signaling. O -fucosylation of nucleocytosolic proteins has not been described previously in any eukaryote and appears to be unique to T . gondii and closely related apicomplexans.