Sporadic and geographically widespread reports of parasites affecting the Belted Kingfisher (Megaceryle alcyon) have been published but few have described details of the pathology. A female, adult kingfisher was found dead in a heavily wooded area of a zoo in Maryland, USA. At necropsy, numerous sexually dimorphic, 4.4–40.5-cm adult Monopetalonema alcedinis nematodes were found tightly wound within the coelomic cavity between organs and completely filling the caudal thoracic and abdominal air sacs. Abundant, 30–60-µm diameter, larvated, thick-walled ova were found in the bronchi and parabronchi, within the mesentery, and in the serosa of multiple coelomic organs. Monopetalonema alcedinis is a characteristic member of the superfamily Diplotriaenoidea, a group of nematodes occurring in birds and reptiles. Infective larvae within an invertebrate intermediate host are ingested and penetrate the intestine, traveling to the lungs and then into the air sacs, where the adult females release eggs. The ova are coughed up by the avian host and passed in feces. Specimens of M. alcedinis have been found in the Belted Kingfisher, although typically intensity of infection is low and infections remain asymptomatic. In contrast, we report the second documented case of high numbers of M. alcedinis resulting in pathologic changes in which parasitism contributed to host mortality.