A certain percentage (34%) of the females in wild populations of the freshwater Isopod Asellus aquaticus (L.) open their marsupia, and about 40% of their embryos are lost in the water. Such an ejection is due to the presence of dead embryos in the marsupium, and this presence is "perceived" by the mother, who gets rid of them operating a true counter-selection. It is possible to interpret the phenomenon as a kind of parental care, in that the female avoids to lose the whole progeny by the removal of the dead. The causes of mortality in embryos have been valued by in vitro culture of the embryos themselves.