 "P. fragilis" was the first species to be described, when Charles Chilton named it "Gammarus fragilis" in 1882; it was moved to the genus "Phreatogammarus" by T. R. R. Stebbing in 1899, becoming the type species for the group.It lives in wells and groundwater, mainly in Canterbury and Nelson, and is not known to occur on the North Island.Chilton's second species, "P. propinquus" was described in 1907 from a single specimen collected near the summit of Mount Anglem on Stewart Island/Rakiura.