Seven species of the nematode genus Mononchus Bastian, 1865, were found in the State of São Paulo, Brazil. Such are: M. subsimilis Cobb, 1917; M. ibitiensis Carvalho, 1951; M. piracicabae Lordello, 1953; M. risoceiae Carvalho, 1955; M. soutoi Carvalho, 1956; M. coronatus Carvalho, 1956; and M. jairi n. sp. Outside the State, the following species have been found: M. papillatus (Bastian, 1865) Cobb, 1916; M. monhystera Cobb, 1917; M. muscorum (Dujardin, 1845) Cobb, 1916; M. gymnolaimus (Cobb, 1893) Cobb, 1916; M. consimilis Cobb, 1917; M. trichurus Cobb, 1917; M. reversus Cobb, 1917; and M. dentatus Cobb, 1917 (LORDELLO, 1953). This field of investigation has scarcely been touched and additional collections will doubtless add new forms to the number of species known as yet. M. jairi n. sp. most closely resembling species is M. monhystera Cobb, 1917, from which it differs in having: a) labial region more expanded, set off from neck by a broad shallow depression; b) anterior portion of the pharynx-wall rather thinner and directed outward; and, c) smaller dimensions (788.0-924.0: 1,100.0 micros).