The phreatic basin of Ljubljansko polje (polje = plain, field) recharges its water supply mainly from the Sava river-bed and at a few other locations where connections with karstic subterranean waters might exist and only up to 15% from precipitation. An important zone of infiltration in the river-bed is the bottom and not the bank which is to a large extent watertight due to organic debris (rests of Sphaerotilus e.g.). The main water-body moves about 10 m/day, there are however some local jets with far higher speeds. Yearly amplitudes of water temperatures are high near the river but in the centre of the plain only a couple of centigrades. Oxygen saturation is in the open river-water 100%, dropping to 40-60% just 1 m into the phreatic. True stygopsammal animals are represented here only by a few species and specimen in spite of the fact, that the interstices in the gravel are mostly filled with finer sediments. Remarkable is also the scarcity of Nematodes and the near absence of Acarina (compare with Danielopol 1976). Only a few specimen of the river benthos (Chironomidae, Tipulidae, Leuctra supp., Baetìs spp.) penetrate the interstitial water (compare with Ruffo 1961, Danielopol 1976) and only Naididae are more frequent there. However, many epigean animals occur in interstitial waters in the periodically flooded gravel-banks; one can explain this with oscillations of the water level. Some epigean animals (creno- and troglophilic) are quite regularly represented in the phreatic near the river, but have not been found in the river-bed. The distribution of phreatic species within the studied water-body seems to be controlled mainly by the presence of food supplies and the consequent competition among species. The same is true for the speed of the water current and some other factors which are less easily defined. The characteristics of the substratum as well as O2-saturation and other characteristics of the water seem to have little influence on the fauna. The energetically (food-) rich neighbourhood of the river is inhabited by a number of species in quite dense populations while the central parts of the phreatic water body exhibit a great poverty of species and of specimen. However, some species live here, which don’t occur in the presence of larger food supplies — and of greater competition (Niphargus serbicus). The higher current speed seems to prevent settlement of some species (Cyclopoida, Proasellus deminutus) while some are bound to such habitats (Proasellus vulgaris). Some species exhibit a high degree of euryvalency inside the stygopsephale habitats (Niphargus longidactylus e.g.), while some are highly specialized. Some of them form dense populations (comparatively dense even in energetically poor places) while others exhibit even in most favourable conditions very low densities (Niphargus jovanovici multipennatus). The present fauna is zoogeographically very diverse. Some species are distributed throughout Europe; some reach from Central Europe to the borders of Dinaride Karst (Bogidiella albertimagni) and some even penetrate it (Trichodrilus pragensis, Acanthocyclops kiefer). Bogidiella semidenticulata, Niphargus pectinicauda, Hadziella deminuta seem to be limited to the higher reaches of the Sava River. All of the above mentioned animals live regularly in interstitial waters and only sporadically in karstic hypogean waters. Niphargus stygius is here the only animal of a certainly karstic provenience; inside the plain it is limited to a completely special habitat. It is very likely that the entire Proasellus-deminutus group has developed in interstitial waters of larger plains which are in contact with karstic areas; some species penetrated from the plains into the karst rather than the reverse. To the contrary (judging from the distribution of the genera) karstic waters seem to be the cradle of Hauffenia and Hadziella. Such a sharp delimitation between cave- and interstitial fauna resp. in this area is very noteworthy. Both faunas live here in abundance and in close contact. It is very probable that particularly high competition and specialization of both faunas, caused by their richness and diversity, prevent mixing of species.