The electrophoretic analysis of egg yolk proteins of the mollusc Octopus vulgaris by both SDS- and native- PAGE, showed two different patterns that mirror the two main periods characterizing the reproductive cycle of the female of Octopus vulgaris: the non-vitellogenic period (October-February) and the vitellogenic period (March-July). Since different levels of progesterone also characterize these stages, we performed in vitro incubations of previtellogenic ovary of Octopus vulgaris with this steroid in order to clarify its possible involvement in the regulation of egg yolk protein synthesis. Treatment of Octopus eggs with progesterone induced the appearance of a 70 kDa protein which cross-reacted with antibodies raised against crab vitellogenin. This protein was further identified as a component of Octopus egg yolk proteins of the vitellogenic period. Bromodeoxyuridine (BrdU) incorporation in the nuclei of follicle cells after treatment with progesterone brought about follicle cell proliferation, suggesting that this hormone may be involved in vitellogeneisis in Octopus.