Abstract
Megabranchiella gen. nov. (Ephemeroptera: Baetidae) is established as a new baetid mayfly genus from northern Thailand. Two new species, Megabranchiellascutulata sp. nov. and Megabranchiellalongusa sp. nov., are described. This genus is distinguished from other Baetidae by abdominal segment I, bearing a pair of enlarged, ventrally oriented single gills, covering abdominal sternites II-V; other gills have normal size and are dorsolaterally oriented. The two new species Megabranchiellalongusa sp. nov. and Megabranchiellascutulata sp. nov. can be differentiated by the setation of femur dorsal margin and the shape of abdominal gill I. This mayfly genus was found in flowing water with cobble microhabitats in headwater streams of northern Thailand.