The remarkable stygobiont mysid Stygiomysis holthuisi (Gordon, 1958) has been originally described from St. Martin (Lesser Antilles), and has been found afterwards also in Puerto Rico. The present paper records it from an anchialine habitat and from a cave on Anguilla, near St. Martin. Comparison of the Anguilla specimens with the available descriptions and with freshly collected specimens from the type-locality gave interesting results concerning the variability of the species. There is generally good agreement between specimens from the three islands, except for the armature of spines of the uropodal pleopodite: in this respect, specimens from Anguilla and Puerto Rico both differ from those caught on St. Martin. Surprisingly enough, the endopodite of the maxilla II was found to be triarticulate, a situation apparently never observed in the Mysidacea.