The underwater Palaeolithic site in the Kaštela Bay near Resnik lies in the immediate vicinity of a well-known underwater site from the Hellenistic period and Roman antiquity, and a Neolithic site has also been discovered nearby. The site was first recognised by Ivo Svilan, who then notified archaeologists. After Z. Brusić first reported on possible Palaeolithic artefacts in our underwater zone over thirty years ago, this is an entirely new case indicating a possible Middle Palaeolithic site. The first archaeological investigations of the underwater Palaeolithic site Kaštel Štafilić took place in September 2008, and also in 2010, 2011 and 2012. Deciding on the methodology for investigating such a site and applying it was a great challenge. The standard method used in investigations of Palaeolithic sites on land was adapted to underwater work, as devised by K. Zupčić and I. Karavanić and taking into account the specific features of the site. In addition to chert debris, whose frequency decreased with depth, a sidescraper and several possible tools were found (most of these are probably pseudo-tools or pseudo-artefacts, partly or entirely created by natural processes). Although the number of artefacts with clearly Mousterian typological features was relatively small, in addition to the artefacts collected earlier by I. Svilan, these proved beyond doubt that this was a Middle Palaeolithic underwater site, the first of its kind in Croatia at which investigations were carried out. The question of the formation of this Middle Palaeolithic site is very complex due to the strong wave action directly influencing the granulometric composition of the sediment, including the artefacts within it. In addition, the finds can be disturbed by various sea animals, as well as man (construction works, e.g. sewer conduits, as well as seabed digging for shells). Although they contain Pleistocene artefacts, the sediments were probably not formed at that time, but later. The finds definitely do not lie in a primary position and therefore should not be considered elements of an open-air Palaeolithic habitation site, unmoved from the time when the sea level was considerably lower than today (the Pleistocene period).