Biokovo Mountain in Croatia is a Dinaric hotspot of subterranean biodiversity and its area of 196 km 2 is protected as a nature park. Until now, more than 400 caves have been registered here, however, only in about 30% of them biospeleological research has been carried out. It is hypothesized that over 1000 caves exist in highly karstified Biokovo Mt. Biospeleological research of the area started at the beginning of 20th century and deep caves were infrequently explored after the year 1980. Previous research recorded more than 90 troglobiotic species in this area and most of them are endemic. The primary goal of the Expedition was a systematic survey of eight deep caves (depth over 250 m), one of which was never before biospeleologically surveyed. We also surveyed other caves from which material for detailed faunistic analysis was missing. The main part of the expedition took place from 17 th to 25 th June 2017, with 52 participants. Fauna was collected by hand and by pitfall traps with baits, in the total of 26 caves. Five deep caves were completely surveyed (depth up to 710 m), the topographic map was made when missing, and fauna was photographed. Microclimatic conditions were measured and in two caves data loggers were placed. According to preliminary results, around 10 new species for science have been found from following groups: leeches (Hirudinea), spiders (Araneae), harvestmen (Opiliones), woodlice (Isopoda), centipedes (Chilopoda), springtails (Collembola), two-pronged bristletails (Diplura) and beetles (Coleoptera). These results confirm that deep caves of Biokovo Mt are still underexplored and that detailed and systematic research is needed.