There are two opposing hypotheses on the origin of Bathynellacea, a marine and a freshwater one. According to the marine hypothesis, Bathynellacea invaded continental groundwater from the sea via the interstitial of marine beaches (“two-step model”), whereas according to the freshwater hypothesis the groundwater was reached from the sea via fresh surface waters and the interstitial of gravelly banks of rivers and streams. According to the marine hypothesis adaptation to life in groundwater happened again and again with practically no subsequent dispersion in this habitat; occurrence in brackish water is primary, that in freshwater secondary; co-occurrence of Bathynellacea with faunal elements of incontestably marine origin is primary and indicative of a marine origin of Bathynellacea as well. According to the freshwater hypothesis there is adaptation to life in groundwater and subsequent large-scale dispersion in it; occurrence in freshwater is primary, that in brackish water secondary; co-occurrence of Bathynellacea with faunal elements of incontestably marine origin is secondary and not indicative of a marine origin of Bathynellacea. These differences are critically discussed with the result that more seems to speak for a freshwater origin. The two-step model with the genusIberobathynellaSchminke, 1973 as an example is scrutinized and found to be incomplete and until now not sufficiently supported.