Nematode faunas in more than 30 mulberry fields in Thailand were investigated to identify nematodes causing damages to mulberry. Criconemella spp., Helicotylenchus dihystera, Hoplolaimus seinhorsti, Rotylenchulus reniformis and Tylenchorhynchus spp. were found to be major nematodes parasitic to mulberry. Ecphyadophora quadralata was found in Thailand for the first time in mulberry fields infected by root rot. Seasonal prevalence of occurrence of five species was examined in the infected fields with root rot at the Nakhon Ratchashima Sericulture Research Center, taking into account their possible relationships with the occurrence of t he root rot. The fact that the peaks of density of H. seinhorsti and E. quadralata coincide with the occurrence of mulberry root rot suggests that these two nematodes be one of the causes of this symptom.