Mode ofFusarium solanipenetration into rough lemon citrus rootstock seedling fibrous roots was determined and root symptomatology was evaluated using light, scanning, and transmission electron microscopy. Roots contained limited subepidermal infection 15 h after inoculation. Invasion of epidermal cells was principally through broken epidermal and root hair cell walls, but direct penetration also occurred. Cortical infection was primarily intracellular. Severe root rot was present 36–60 h after inoculation and in most cases infection after 60 h resulted in disruption of the phloem and endodermis around the still relatively intact primary xylem. Naturally infected roots from field trees were collected in an advanced stage of rot. In these,F.solanidigestion of middle lamellae of primary xylem was common, and infection and erosion of secondary vessel element walls occurred frequently.