A new species, Sensonator valentiensis, the only representative of a new subterranean amphipod genus, was discovered in a rather small area in the S.E. part of the province of Valencia. A few specimens have been found in wells, a hyporheic habitat, and a phreatic cave lake. In general, Sensonator has a rather primitive morphology with remarkable features such as presence of calceoli on the antennae in both sexes, reductions in the mouthparts, and a natatory third uropod. The new genus is very distinct from other stygobiont amphipods, and it cannot even be classified in one of the existing families of Gammaridea, but with some reserve it is placed close to the Gammaroidea and Pardaliscoidea. S. valentiensis was not attracted by baits, but has been seen grasping prey; the animal is observed to be a good swimmer, although it also creeps on muddy substrates. Its favoured biotope is assumed to be large fissures in the saturated zone of the karst; from there it seems to have colonized macroporous alluvia. Active colonization from the marine environment into the littoral karst during Miocene, or possibly Late Cretaceous, is proposed as evolutionary scenario.