We report the first observations of Opheodrys vernalis (Smooth Greensnake) communal oviposition of 137 eggs within an active nest mound of Lasius interjectus (Citronella Ant) in a northern Illinois prairie. This is the first North American documentation of snake oviposition, and communal oviposition, within an active ant nest. The communal Smooth Greensnake nest is also the largest reported to date. We observed other instances of Smooth Greensnakes nesting in proximity to Citronella Ant nests beneath artificial cover objects, which may indicate a facultative inquiline relationship. Other documented North American associations between snakes and ants have been with Formica mound-building ants in which snakes used mounds as refugia or hibernacula. The only other North American observation of a reptile nesting within an active ant mound was with Anolis carolinensis (Green Anole) and Odontomachus brunneus ants in Florida (Kwapich 2021). Other accounts of inquiline oviposition by snakes within active ant nests have been from Central America, South America, and Africa, predominantly within the fungal-farming chambers of leaf-cutter ant genera Acromyrmex, Anochetus, Apterostigma, Atta, Camponotus, Platythyrea, and Strumigenys.