Samples of larvae, metamorphosing and adult representatives of anadromous Petromyzon marinus were collected from rivers in New Brunswick to provide data on aspects of its life cycle. The proportional length of the prebranchial, branchial, trunk and tail regions changed throughout larval life, with the third of these lengths being greater than in Lampetra species, a feature probably related both to the higher number of trunk myomeres and to the greater number of oocytes found in the Sea lamprey. Length-frequency curves for ammocoetes indicate that in general the duration of larval life is from six to eight years. Metamorphosis is initiated in mid-July and has probably been completed by November, paralleling the situation in the landlocked Sea lamprey. Small adults of variable size (167–351 mm) attack Alewives (Pomolobus pseudoharengus) and Atlantic salmon (Salmo salar) in the St. John River at points up to 120 km from the estuary. The data indicate that some individuals start feeding in the winter while others do not commence parasitism until the late spring and that this species does not always migrate downstream to saline conditions immediately after metamorphosis. In a sample of 285 nearly mature upstream migrants caught at the Mactaquac Dam on the St. John River, the ratio of females: males was 1: 1.18. No significant difference was found between the mean length, weight and number of trunk myomeres of the two sexes with respective values of 72.9 cm, 892.1 g and 70.1 being recorded for the females and 72.2 cm, 872.6 g and 69.6 for males. The gonadosomic ratio in males was 1.6 and 14.7 in females. The mean number of eggs per female was 210,228 with a range of 151,836 to 304,832. Spawning takes place in late June/early July when the animals show a sexual dimorphism in body proportions and undergo a reduction in total length. The post-larval phase of the life cycle is considered to last for three or four years, one or two years longer than that recorded for the landlocked form.