Metamorphosis of the nonparasitic mountain brook lamprey, I. greeleyi, is described within 7 phases based on external morphological changes. The commencement of metamorphosis in early August is consistent with the pattern for northern hemisphere lampreys which display an inverse relationship with latitude. The sequence of external changes including the development of eyes, oral disc, teeth, tongue, oral fimbriae and lateral line its consistent among individuals of I. greeleyi and lampreys in general. Some of the changes realized during metamorphosis of I. greeleyi are important to the adult period while others reflect their presumed parasitic ancestry. Energy reserves by nonfeeding I. greeleyi during the 104–140 days required to complete metamorphosis are replaced by water so that total length and body weight do not change significantly during this time.