The development of hatchery production techniques for blue mussels is receiving increased research attention in Europe as it could offer an alternative source of spat for the industry ('Blue Seed' Project, Kamermans ICSR 2007). Among the necessary techniques are those of controlled spawning and fertilization, and techniques of triploidisation that can offer increased seasonal availability, yield and/or decreased growing time in the subsequent production stages. In several experiments, Mytilus edulis oocytes, obtained by thermally induced spawning, were fertilized with Mytilus edulis spermatozoa obtained in the same way or by stripping. Parallel treatments examined the effect of mixing Mytilus edulis oocytes with spermatozoa stripped from the Pacific oyster Crassostrea gigas. Initially considered as a potential stimulant for mussel spawning, the sperm of C. gigas was actually found to activate mussel oocytes, which were observed to slowly expulse their 1st and 2nd polar bodies. This form of activation was repeated in four separate trials in 2006 and 2007. No reciprocal effect was observed when oyster oocytes were mixed with mussel sperm. Examination of the embryogenic stages by epifluorescence revealed that though C. gigas sperm attached to the mussel oocytes, no penetration occurred. Resulting embryos would therefore be haploids. No viable haploid D larvae were obtained by this technique while the treatments with mussel sperm (spawned or stripped) produced good larval yields. Such a means of oocyte activation without fertilization could however be useful for induction of gynogens, and a later polar body retention experiment produced small numbers of D larvae. The capacity of oyster sperm to activate and thus bring about a reduction in available mussel oocyte numbers might also have implications for the cohabitation of these two species, notably in the context of C. gigas as an expanding invasive species in Northern Europe. The main spawning periods of these two species are different in the wild (i.e. early spring for mussels, summer for oysters) but climatic change and indications of expanding spawning period in both species may increase the likelihood of their gametes coming into contact in the future.