The lengths, weights and condition factors of young adult lampreys caught feeding on Gaspereau, Alosa pseudoharengus. Shad, Alosa sapidissima, and White suckers, Catostomus commersoni, in the lake-like extensions of the St. John River system during May, suggest that many lampreys do not feed in the nine to ten months following the initiation of metamorphosis in mid-July. The mean lengths (d-95% confidence limits) of individuals taken in four samples between May 13 and 29 did not differ significantly and ranged from 132-7± 2-81 to 135-7 ±3-71 mm. A significant increase in mean condition factor from II15 to 1-353 during this time indicates that the short period spent feeding in May enables the animal to replenish its food reserves prior to its downstream migration. Since several of the 81 feeding adults caught between the latter part of June and mid-July, at a point 140 km from the estuary of the St. John, were still attached to spawning-run Atlantic salmon, Salmo salar, this teleost host had probably been responsible for their transport upstream. The wide range in their length (156-403 mm), together with the presence in the lakes in May of a few young adults larger than those which have only just started feeding, suggest that the time of onset of the parasitic phase is variable but may generally commence in either the late autumn or in the spring. Scarring on salmon due to lamprey attack was predominantly (84%) on the ventral surface between the operculum and caudal peduncle. Over 35 % of the salmon examined in June 1975 had been attacked, more of these attacks being on the right side of the body of both males (59-6%) and females (49-8%) than on either the left (20-2 and 23-7 %) or both sides (20-2 and 26-5 %). While lamprey attacks may cause some mortality among the smaller teleosts, any effect on salmon is probably indirect through causing increased susceptibility to infection and stress. The ability of adult anadromous Sea lampreys to feed and grow in fresh water demonstrates the relative ease with which landlocked forms could have been evolved in response to extreme environmental changes. This evolutionary step has involved a decline in the ability to osmoregulate in high salinities and a reduction in body size and fecundity, the adaptive significances of which are discussed. Data on upstream migrants indicate that males and females undergo length reductions of at least 11 and 15 % respectively between the time of their entry into fresh water and the completion of spawning.