Experimentally transmitted Ascarophis sp. (Spirurida) developed to adult worms in the invertebrate host, Gammarus deubeni (Amphipoda), collected in the intertidal zone in Passamaquoddy Bay, New Brunswick, Canada. The morphological development and growth of larval stages is very similar to other cystidicolids, which are found as adults in fish. Unlike virtually all other Spirurida, which require a vertebrate definitive host, infective larvae of Ascarophis sp. migrate from the invertebrate host musculature into the hemocoel where they molt twice to become adults. Gravid females appear at 80 days and 69 days post-infection at 10-12 C and 18-20 C, respectively. While there is little evident host reaction to the parasite within the muscle tissue, within the hemocoel there is hemocytic reaction to shed nematode cuticles, released eggs, and sometimes the worm itself, including some melanization. The worms are morphologically similar to Ascarophis sp. from G. oceanicus in the Baltic and White seas and among Ascarophis species from fish is most similar to A. arctica. It is suggested that Ascarophis sp. no longer requires a vertebrate host and is transmitted between amphipods either through death and disintegration of infected amphipods and dispersal of the nematode eggs, or more likely through cannibalism or necrophagy.