Objectives: Focal cortical dysplasia (FCD) II is a frequently observed histopathological substrate in epilepsy surgery. In the present study, we explored the spatial distribution of epileptogenic activities across FCD II lesions using stereoelectroencephalography. Methods: Patients with histopathologically confirmed type II FCDs and who had at least one depth electrode that go through the wall of the dysplastic sulcus from the surface to the bottom were included. The dysplastic sulci were divided into the bottom and non-bottom parts manually, and contacts were defined as bottom or non-bottom contacts according to their locations. Factors (bottom location, pathological subtype, magnetic resonance imaging manifestation, and presence of bottom-of-sulcus dysplasia) potentially associated with earliest onset identified by conventional visual analysis, epileptogenicity index (EI), and standardized number of high-frequency oscillations (HFOs) were analyzed. Linear regression analyses between distance (from the location of the analyzed contact to the bottom of the sulcus) and EI value and HFO number were performed. Results: Sixteen patients with 19 depth electrodes containing 112 valid contacts were included. Bottom location was the sole factor significantly associated with earliest onset (P < 0.001), EI value (P < 0.001), and HFO number (P < 0.001). Most earliest onsets were recorded by the bottom contacts, bottom contacts had higher EI value (0.81 +/- 0.28 vs. 0.31 +/- 0.24, P < 0.001) and more HFOs (0.78 +/- 0.28 vs. 0.35 +/- 0.31, P < 0.001) than non-bottom contacts. Moreover, the EI value (R = -0.72, P < 0.001) and HFO number (R = -0.64, P < 0.001) were significantly negatively correlated with distance, regardless of histopathological subtype, MRI manifestation, or absence of bottom-of-sulcus dysplasia. Conclusion: Seizure onsets and interictal HFOs most often arise from the bottom part of a sulcus with type II FCD. Significance: The findings of the present study contribute to intracranial electrode selection, trajectory planning, and, later on, resection of this kind of malformation. (C) 2019 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. All rights reserved.