Fertilization is external in some crustaceans and internal in others. In the nineteenth century, it was reported that the American lobster (Homarus americanus) fertilized its eggs externally using spermatozoa stored in the seminal receptacle. Later investigators questioned this assumption, noting that the spermatophoric mass is sealed inside the seminal receptacle by an impervious sperm plug that prevents the spermatozoa from exiting the seminal receptacle via the orifice. This led to suggestions that fertilization in homarid lobsters is internal via transitory connectives that form between the seminal receptacle and the ovary/oviduct at the time of oviposition. Supporting evidence was obtained from studies in which fertilization occurred even when the orifice of the seminal receptacle was sealed with epoxy. We here report selective occlusion studies that resolve this conundrum. At spawning, spermatozoa leave the seminal receptacle not through the sealed orifice, but via two grooves located posterior and lateral to the orifice. Occluding these posterolateral grooves as well as the orifice of the seminal receptacle prevents fertilization, whereas occluding only the orifice does not. This explains why earlier attempts to prevent fertilization by occluding the orifice of the seminal receptacle were unsuccessful, and confirms that fertilization in the American lobster is external.