The diversity of statoliths and changes in statolith length were examined in larval and metamorphosing sea lampreysPetromyzon marinasfrom four streams in Ontario. In midsummer, increases in statolith lengths were similarly and positively correlated with larval total lengths in the four streams. Statoliths from larval and metamorphosing lampreys collected from Lynde and Farewell creeks in June and September displayed typical alternating opaque and translucent bands. The number of opaque bands, or annuli, provided reliable age estimates when compared with length-frequency distributions. In July and September, statoliths from some larval and metamorphosing lampreys collected from West Root River and Cannon Creek were either absent or did not have typical bands, hence they did not always provide reliable ages. The diversity of statoliths appears to be related to ambient calcium ion concentrations, especially during periods of rapid larval growth. The use of statoliths is sometimes the only method to age some populations of sea lampreys because of ambiguity in length-frequency distributions. The absence of statoliths, as encountered in this study, has potential management implications when determining age-at-metamorphosis.