Two new genera of free-living marine nematodes are described from the muddy bottom of Cienfuegos Bay, Caribbean Sea. Guitartia n. gen. (Xyalidae, Monhysterida) is characterised by three long tooth-like structures in the stegostom, second and third circle of anterior sensilla separate and posterior genital branch of the female restricted to a long post-vulvar sac. The type species is Guitartia tridentata n. gen., n. sp. Within the Xyalidae, Guitartia n. gen. is morphologically close to Amphimonhystrella, Cobbia, Elzalia, Scaptrella and Valvaelaimus, all being characterised by sclerotised structures in the stoma and transverse striation of the body cuticle. Main features for discrimination are the type of stoma structure, the shape and relative size of amphidial fovea, the presence of a post-vulvar sac and gubernacular apophyses and the absence of terminal setae on the tail. Macrodontium n. gen. (Microlaimidae, Chromadorida) is characterised by a heavily sclerotised stoma with one large dorsal tooth and two smaller subventral teeth, sexual dimorphism in size and position of the amphidial fovea and males with a single anterior testis. The type species is Macrodontium gaspari n. gen., n. sp. Within the Microlaimidae, Macrodontium n. gen. is similar to the genera Acanthomicrolaimus and Bolbolaimus due to sclerotised stoma and presence of large dorsal tooth. The new genus is similar to Aponema in sexual dimorphism in the size of the amphidial fovea, monorchic males, presence of a gubernacular apophysis and conico-cylindrical tail shape. Morphological characters of diagnostic value within the family are the ornamentation of the body cuticle, relative length of cephalic sensilla, stoma sclerotisation and number of testes.