The distribution of Bolbelasmus unicornis (Schrank, 1789) is critically reviewed throughout its range with emphasis on the Czech Republic and Slovakia. The species has been reliably recorded from 377 localities in 19 countries. New records are given from 152 localities of Bulgaria, Czech Republic, Germany, Hungary, Italy, Moldova, Poland, Romania, Serbia, Slovakia, Turkey, and Ukraine. For Germany, the species is recorded for the first time in 54 years. The occurrence of the species in Switzerland is confirmed by two historical specimens from Zürich. The only known historical specimen labelled “Kaukasus” is given, which could originate from Russia, where this species has not been recorded before (however, confusion of the locality label cannot be ruled out). All published faunistic data from across the range are presented here in full, in several cases supplemented by details subsequently obtained by the author. Distribution maps are compiled separately for the Czech Republic and Slovakia, and for the entire range. A separate map is also available for Hungary, where approximately one-third of the known localities are located. Statistical data concerning the flight activity of adults, seasonal dynamics for part of the distribution area, details of records and notes on the bionomy and ethology of the species are provided. Possible feeding strategies for adults and larvae of B. unicornis are discussed, as well as current knowledge of the natural history of various representatives of the subfamily Bolboceratinae. A monitoring method for the species is proposed.

The author provides corrections of minor errors and omissions from his initial study, as well as data from omitted and new literature, and new records based on the material studied and new observations. For some of the previously published records, details obtained subsequently by the author are added. The first record of Bolbelasmus unicornis for Belarus is given, representing the northernmost known occurrence of the species. The second recent record for Croatia is quoted from an internet source. Updated distribution maps are provided for the Czech Republic and Slovakia, and for the entire range, as well as a distribution map of the Western Palaearctic representatives of the B. unicornis species group. The species is currently known from 386 localities in 20 countries.