A new stygobite species Acanthocyclops transylvanicus sp. nov. is described from the north-western Romanian Carpathians (Apuseni Mountains) and is accommodated in the subterranean kieferi-group. Acanthocyclops transylvanicus sp. nov. resembles closely A. biarticulatus Monchenko, 1972 in the segmentation pattern of the swimming legs (3.2/3.2/3.2/3.2), but differs by the following characters: antennary second endopodite segment with 7 setae; distal articles of endopodites of P1 and P3 with 4 and 5 setae, respectively; absence of coxopodite ornamentation pattern in leg 4; leg 4 endopodite with 3-segmented appearance, but lacking a functional articulation between the second and third segment. We assume that the 2-segmented nature of the leg 4 endopodite as observed in both species is a convergent acquisition. The adult shape of the leg 4 endopodite in A. transylvanicus sp. nov. results from the simple suppression of an arthrodial membrane formation. A. transylvanicus is the ninth representative known in Romania which is accommodated in the kieferi-growp. An identification key for the species of kieferi-growp known to occur in Romania is given.