Abstract: Steroidogenesis, which plays a major role in the reproductive cycle of vertebrates, is still for the most part, unknown in invertebrates. The aim of this study was to examine the link between progesterone and the reproductive cycle in Mya arenaria. The soft-shell clams, Mya arenaria were collected in Anse à l'Orignal (Parc Provincial du Bic, Québec, Canada) from July to November 1998. Histological data have shown that female gonads of M. arenaria were in the spawning stage in August and September, while the male gonads were in the ripe stage. This period of active gametogenesis was associated with a depletion of lipid reserves. These lipids could be used as a source of energy and as a substrate for steroidogenesis. Liquid Chromatography coupled to Mass Spectroscopy (LC-MS) and quantified by Enzyme Linked ImmunoSorbent Assay (ELISA) determined progesterone. Progesterone levels in the gonad were increased during the ripe stage in the male and during the spawning stage in the female. These results indicate, for the first time, that progesterone, as in vertebrates, may play a role in the reproductive cycle of M. arenaria.