Samples of Nephrops norvegicus were collected on a monthly basis from October 1993 to September 1995 in different areas of the Mediterranean and the Atlantic, both to investigate molt periodicity and to verify the existence of a common molt pattern for this species in the Mediterranean. The presence of well-developed gastroliths in the wall of the stomach was used as an indicator of molting periods. A well defined molting periodicity was not found among juveniles, which seem to molt all year round. In each studied area molt synchronism was evidenced among both males and females after the onset of sexual maturity, even if the seasonal molting pattern differs between the two sexes. Adult females have only one molting period per year (December-March), immediately after the hatching of eggs. Adult males present a molting period in late summer-autumn (August-October). In some areas a second molting period is evident and co-occurs with that of females (late winter-early spring). This molt synchrony among adults is common to all the Nephrops populations of the Mediterranean Sea. The different results obtained for the Atlantic population could be due to the influence of environmental factors on molt cycle regulation.