The Devonian is a period that recorded many biocrises. One of them, known as the Kačák event ranges through the Polygnathus ensensis Zone immediately before the Eifelian-Givetian boundary (Middle Devonian). This crisis has been identified in many localities worldwide, mostly in bathyal settings. The event, divided into two phases ( otomari event and Kačák event s.s .), is typically marked by a turnover among pelagic faunas, especially conodonts, dacryoconarids and ammonoids. This turnover result from a transgression associated with anoxia that generally corresponds to the deposition of black shale in deep-water settings. In Belgium, the Kačák event s.s . corresponds to a time window equivalent to the deposition of the Lomme and Hanonet formations. New stratigraphic and palaeontological (rugose corals) data show that the Kačák event had a moderate to weak impact on the Belgian carbonate shelf ecosystem. The Old World Realm faunal assemblages show no significant variation in diversity across the Kačák event. Nevertheless, the remarkable and unexpected occurrence of some rugose corals typical of the East American Realm (siphonphrentids and heliophyllids) in the lower part of the Hanonet Formation helps identify the event as the latter is also marked by a short phase of cosmopolitanism of benthic fauna. This is a proposed criterion useful to recognize the Kačák event where the typical pelagic guides are missing.