A new nematode species, Philometra thaiensis sp. nov., is described from gravid female specimens collected from the abdominal cavity of two freshwater Tetraodon spp., T. palembangensis (type host) and T. fluviatilis (Pisces, Tetraodontidae), after their import from Thailand into the Czech Republic in October 1998. It is characterized mainly by its body length (40.46-49.34 mm), arrangement and number of small cephalic papillae (eight papillae in outer circle and six papillae in inner one), structure and length (0.84-1.40 mm) of the oesophagus, short ovaries, and the absence of caudal projections. A key to Philometra spp. with gravid females parasitic in the abdominal cavity of fishes is provided.