In soil, moss and freshwater habitats of tropical forests in Costa Rica, 20 mononch species were found. Two are described as new species. Mononchus laminatus n. sp. (L = 1.5-1.9 mm, buccal capsule 29-31 × 11-14 μm, c′ = 3.6-4.9, similar to M. aquaticus but with a subventral lamina present in anterior part of buccal cavity, and amphid aperture posterior to the dorsal tooth) and Miconchus gomezi n. sp. (similar to M. digiturus, but with slender body, vulva cuticularised and with a reduced posterior genital branch). The known species are redescribed, the well-known ones briefly, the less known more extensively. Brazilian and Costa Rican specimens of Paracrassibucca paucidentata Lordello, 1970 were studied and a lectotype has been designated. The other species found are: Mononchus truncatus Bastian, 1865; M. aquaticus Coetzee, 1968; M. tunbridgensis Bastian, 1865; Coomansus parvus (de Man, 1880); Coomansus sp. (C. zschokkei-group); Prionchulus muscorum (Dujardin, 1845); P. punctatus Cobb, 1917; Cobbonchus coetzeeae Andrássy, 1970; Mylonchulus contractus Jarirajpuri, 1970; M. hawaiiensis (Cassidy, 1931); M. obtusicaudatus (von Daday, 1899); M. parabrachyuris (Thorne, 1924); M. sigmaturus Cobb, 1917; Mulveyellus monhystera (Cobb, 1917); Iotonchus tenuidentatus (Kreis, 1924); I. trichurus Cobb, 1917 and Miconchus digiturus (Cobb, 1893).