Two new species of nematodes associated with crabs are described from New Caledonia. Gam-marinema scyllae sp. n. from the gill chambers of Scylla serrata (Forsskal) is characterised by 3–4 mmlong body, small outer labial and cephalic sensilla, distinct ocelli, short straight spicules and sub-cylin-drical tail. Monhystrium mangrovi sp. n. from the gill chambers and body cavity of mangrove crab Neosar-matium sp. is characterised by 1–1.4 mm long body; outer labial sensilla longer than cephalic sensilla,amphid located at level with posterior stoma chamber,denticles in posterior stoma chamber and five pairs ofgenital papilla on tail. Phylogenetic relationships oftwo new species and other nematodes from the familyMonhysteridae are analysed based on 18S and partial28S rDNA sequences.