The majority of oligochaete species living in springs are ubiquitous and widely distributed taxa but cold stenothermic, semiaquatic and rare ones are also found. In Poland about 50 species are known from springs among 185 aquatic and semi-aquatic species found in whole country. Gianius aquaedulcis (Hrabĕ 1960) (Tubificidae, Phallodrilinae) is reported for the first time from Poland from two karstic springs situated in Kraków – Częstochowa Upland (South Poland). Both springs are hillside, fissure, descending, rheocrenes, with discharge about 30– 40 L s⁻¹. Spring no 1 is situated in banked pond, spring no 2 has natural character. Their bottom is sandy or sandy-gravely, with patches of detritus, mud and plants. The conductivity, concentration of main cations (Ca, Mg) and anions (Cl, SO₄) were typical for spring waters of this limestone area; only concentration of nitrate is slightly elevated. The samples of bottom fauna (a polyethylene corer and a bottom scraper) and water for chemical analysis were taken seasonally. This species was more numerous in a banked spring than in the natural one (maximal density about 1600 ind. m⁻²) and seasonal changes of its density were similar in both springs. Mature specimens formed only 5–8% of the population and were present in all sampling data. G. aquaedulcis is known from a few localities only (Germany, France, Spain and USA). It lives in underground and surface waters, with slightly alkaline pH on sandy or stony bottom. Maybe this species forms a few relictic populations which persist in subterranean (cave or hyporheic) waters of some geographic regions at least since the last glacial epoch. The existence of small anatomical differences (for example various thickness of the muscular lining of the atria and changeable shape of penial setae) among the studied populations from various countries seems to confirm this hypothesis.
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