The morphology of infection structure development of Puccinia recondita f. sp. tritici on and in susceptible and resistant wheat lines inoculated with urediospores was examined by SEM. The germ-tube extends over the leaf surface and elongates perpendicularly to the long axis of the leaf. When the germ-tube encounters the stomatal lip, an appressorium forms over the stoma and the pore is entered by an infection peg produced on the surface of the appressorium in contact with the host leaf. At 6 h post-inoculation (hpi), infection pegs develop terminally substomatal vesicles (SSVs) in the substomatal chambers of all wheat lines. A septum separates each SSV from its interconnective tube. A primary infection hypha forms terminally from the elongated SSV either parallel to the long axis of the stomatal slit or perpendicular to the leaf surface. When a primary infection hypha attaches to a host cell, a septum forms cutting off the tip of the hypha, delimiting a terminal haustorium mother cell (HMC) by 12 hpi. Secondary infection hyphae arise from a position proximal to, and in the proximity of, the HMC septum. Additional HMCs are formed when a secondary hypha or a tertiary hypha adheres to a plant cell. Infection sites with HMCs were observed at 24 hpi and at subsequent sampling stages. There were no significant differences between the infection processes on the three wheat lines examined in this study.