A short account on some achievements of the cubano-romanian biospeleological expeditions to Cuba in the study of the aquatic subterranean faunas. The following divisions of the aquatic subterranean realm are reviewed together with their most characteristic faunal elements: “guano pools” and rimstone pools in the vadose zone of the caves; underground streams; water table (and other) lakes in the caves; “pozzos” carved in the limestone, and “grietas” which are vertical clefts in the limestone of marine terraces, giving access to fresh- or to brackish water; the interstitial of the marine beaches; the underflow of running waters. At present, thorough biospeleological research is being carried out almost everywhere in Central America; Cuba, which remained until recently rather poorly investigated, proves to be one of the most remarkable areas from this point of view. A few of the most interesting problems rose in the course of the study of the underground aquatic fauna of Cuba are listed. An interesting biogeographical problem is the following: some of the subterranean aquatic elements prove to be related to elements belonging to the fauna of the other Antilles and of Mexico, but not to the South-American fauna (as is the case for some terrestrial groups). The research undertaken will be a contribution to the problem of the divisions of the aquatic subterranean realm and of their reciprocal relations, in a warm and humid climate; it will also contribute an answer to the problem of the differences between temperate and tropical cave communities; finally, it allows one to perceive in its very progress the process of colonization of the subterranean freshwaters by elements of marine origin, either through the interstitial realm or through the fissures of the littoral limestones.