The present paper is the sixth of a series summarizing the aquatic coleopterous fauna of Nevada. The introductory remarks prefacing the treatment of the Nevada Dytiscidae (La Rivers, 1951) apply to the other groups as well. The Hydrophilidae, exceeded in number of species among aquatic Coleoptera only by the Dytiscidae, is a dominant element in any 'body of water containing a varied beetle fauna, and the majority of species are vegetarians. Carnivorous tendencies seem to be generally restricted to the larger forms, and the giant of the family, Hydrophilus triangularis, plays a part equally important to that of Dytiscus in the economy of most fishponds. Unlike dytiscids, which are all aquatic, many hydrophilids are terrestrial, one prominent segment of the family being composed entirely of dung inhabitants, and all gradations between the strictly terrestrial and the strictly aquatic types occur, the intermediate species b-eing found along the edges of water. Swimming efficiency of adult hydrophilids is high, but inferior, generally, to that of dytiscids, since streamlining is slightly less pronounced, and the swimming stroke is a trot, i.e., an alternate motion, rather than the more competent gallop or unison motion of dytiscids. Some hydrophilids seem never to swim, but are crawlers on vegetation, rocks, etc. The respiratory reservoir in adults is the usual space between elytra and abdominal tergites, and because of the generally greater curvature of the former in this family, is quite large. Whereas the process of renewing air in the reservoir is a simple matter in dytiscids, and quickly executed, hydrophilids have a somewhat roundabout method of accomplishing this. In general, they contact the surface film with the anterior angle of the pronotum, and the club-shaped antennae alternately sweep out of water and back again, on each descent carrying an air bubble entangled in the hairy club. Some of this air is used to maintain the thin hydrostatic film on the venter, the remainder traverses a groove lying between the mesoand metathorax to the air reservoir. A corollary development of these structural modifications is the enlargement of th.e mesoand metathoracic and the first abdominal spiracles. Like dytiscids, hydrophilid larvae are nearly all air breathers and must come to the surface periodically to renew their supply, and only an occasional species possesses the ability to extract oxygen directly from the water (viz., Berosus, etc.) The tracheae are terminal in position, opening in the bottom of a transverse groove, and are closed by cercal muscles much as in the dytiscids; when the cerci are extended, the tracheal openings close, opening again only upon the flexion of the cerci. Cercal flexure also greatlyaids the larva in maintaining its position at the surface by extending the cerci horizontally along the surface film and giving the body support.