During rescue excavations carried out near the vicus at Kempraten (municipality of Rapperswil-Jona, St. Gallen, Switzerland) in advance of a private construction project, a Mithraeum measuring approximately 8 by 10 m was unexpectedly discovered in the summer of 2015 and subsequently excavated and investigated in detail. This paper presents the preliminary results of the excavation, which was completed less than a year ago, and pays particular attention to the interdisciplinary approach used in the excavation. These included intense sampling of the features for the purposes of micromorphology and archaeobiology. Three construction phases with intermittent conflagrations were identified. The question as to whether there was an ante-chamber remains unanswered. The external areas are also quite difficult to interpret, at least for the time being. The rich assemblage of finds, which included numerous coins, pottery, animal bones and a range of religious artefacts (e.g. altars and a half relief), will only be dealt with in a cursory manner here. According to the range of coins, the Mithraeum undoubtedly dated from the late 3rd to the late 4th or early 5th centuries. The site will be analysed by an interdisciplinary team and preliminary work is already underway.