The shape of sternal calcium carbonate deposits (CCD) preceding moult was used as a diagnostic character to distinguish between reproductive and non-reproductive females in six species of terrestrial isopods belonging to different families. Samples in the wild at the beginning of the reproductive season revealed females with CCD of incomplete shape, with a diamond-shaped opening at the level of central furrow. All the females showing this pattern made a parturial moult a few days later, while all females showing complete CCD made a normal moult. A laboratory experiment confirmed that these characteristics were diagnostic for females performing a secondary vitellogenesis. These females were receptive to mating at that time, provided the stage of the intermoult cycle was not too early. Screening the CCD shape is therefore a non-destructive method that can be used for routine diagnosis of females' reproductive status in terrestrial isopods.