ABSTRACT: Although many marine invertebrates reproduce by releasing gametes into the water, major spawning events have rarely been observed for cold-water invertebrates. We documented mass spawnings by the sea star Asterias vulgaris and the ophiuroid Ophiopholis aculeata on 6 July 1996 and 2 July 1998, and by these 2 species plus a second ophiuroid Ophiura robusta and the sea urchin Strongylocentrotus droebachiensis on 3 July 2003. In each year, the abrupt spawning events coincided with the first major intrusion of warm surface water into a region normally dominated by cold upwelling. The behaviours of the ophiuroids and sea stars favoured fertilization success. First, they spawned from elevated surfaces, with the central disc raised, to increase suspension and mixing of gametes. Second, many individuals pseudocopulated, which involved crawling over one another in the case of the sea star, and individuals holding their central discs together in the case of the 2 ophiuroids. Models of fertilization success need to incorporate such behaviours to better understand fertilization in the sea.