A study has been made of metamorphosis in anadromous sea lampreys in New Brunswick between 1973 and 1976. The stages (1–7) commence with the recently described 'prometamorphic ammocoete' (Youson, J. H., G. M. Wright, and E. C. Ooi. 1977. The timing of changes in several internal organs during metamorphosis of anadromous larval lamprey Petromyzon marinus L. Can. J. Zool. 55: 469–473.) and conclude with one similar to the newly metamorphosed landlocked sea lamprey (Manion, P. J., and T. M. Stauffer. 1970. Metamorphosis of the landlocked sea lamprey, Petromyzon marinus. J. Fish. Res. Board Can. 27: 1735–1746.). Most of the marked quantitative changes exhibited by certain regions of the body can be directly related to adaptations for suctorial feeding, increased activity, and a predaceous mode of life. Reductions in the prebranchial length are almost certainly associated with changes in the pharyngeal basket which facilitate alterations in the direction of the respiratory water flow. The timing of metamorphosis was remarkably synchronous both within and between years, representatives of the earliest stages (1–2) being found only between July 7 and 28 while after September 14 only stage 7 animals were caught. During metamorphosis the lengths of the animals changed little whereas their weights declined significantly. The weights and condition factors of stage 1–2 animals were significantly greater than those of larvae of comparable length. This feature, presumably related to lipid accumulation at the end of larval life, is discussed in the context of factors influencing the onset of metamorphosis.