We tested the hypotheses that the incidence of metamorphosis in sea lampreysPetromyzon marinusis related to the magnitude of the spring rise in temperature and that high larval density prior to metamorphosis reduces the number of animals entering metamorphosis. Concurrently, we also tested the accuracy of predicting metamorphosis based on minimum size criteria of 120 mm and 3.0 g and condition factor (CF) of 1.45 for larval data collected in the fall, where CF = 106(mass, g)/(total length, mm)3. The spring rise in temperature was a critical step in stimulating metamorphosis, but neither the magnitude of warming in the spring nor larval density significantly affected the proportion of metamorphosing animals. By using a CF of 1.45 or greater as the criterion for larval data collected in fall, we predicted 92% of the metamorphosis that occurred the following summer. Predictions of sea lamprey metamorphosis based on fall data were as accurate as predictions made in previous studies that examined spring data and used minimum size criteria of 120 mm and 3.0 g and a CF of 1.50 or greater. The equal success of using a CF of 1.45 or greater with fall data and a CF of 1.50 or greater with spring data might have reflected that some lipid accumulation occurs between the fall and the onset of metamorphosis the following July.