<p>Pacific lamprey, an anadromous fish native to the North Pacific Ocean, are imperiled in parts of their range. Many aspects of their biology are not fully understood, including the physiology of their spawning migration. Pacific lamprey exhibit two divergent reproductive life histories: early-maturing and late-maturing. Interior Columbia River early-maturing lamprey complete the final stages of gonadal maturation ~ 10&#xa0;months after entering freshwater. From the same migrating cohort, the late-maturing lamprey wait ~ 22&#xa0;months to complete maturation. This study sought to characterize the reproductive phenotypes of these two Pacific lamprey life histories. Migrating adult Pacific lamprey that had entered fresh water were collected and held for close to two years to monitor changes in morphology and physiology. The lamprey in this study, of approximately equal sex ratio, clearly separated into early- or late-maturing life histories based on differences in timing of reproductive development, morphology of length, weight, interdorsal fin gap, and plasma concentrations of estradiol-17β (E2), but not plasma androstenedione. Average plasma E2 levels in females and males were significantly higher in the early-maturing fish compared to the late-maturing fish by November, nearly half a year before any morphological divergence. This novel physiological indicator could benefit restoration activities (translocation and artificial propagation) by providing advanced identification of the two maturation life histories present in the same migration cohort. In conclusion, this study provides several morphological and physiological features that can be used to identify early- and late-maturing Pacific lamprey within the 10&#xa0;month period prior to when the early-maturing fish mature.</p>