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The environmental history of American eels Anguilla rostrata from the East River, Nova Scotia, was investigated by electron microprobe analysis of the Sr: Ca ratio along transects of the eel otolith. The mean (±SD) Sr: Ca ratio in the otoliths of juvenile American eels was 5.42 x 10-3±1.22 x 10-3 at the elver check and decreased to 2.38 x 10-3±0.99× 10-3 at the first annulus for eels that migrated directly into the river but increased to 7.28 x 10-3±1.09 x 10-3 for eels that had remained in the estuary for 1 yr or more before entering the river. At the otolith edge, Sr: Ca ratios of 4.0 x 10-3 or less indicated freshwater residence and ratios of 5.0 x 10-3 or more indicated estuarine residence. Four distinct but interrelated behavioural groups were identified by the temporal changes in Sr: Ca ratios in their otoliths:(1) entrance into freshwater as an elver,(2) coastal or estuarine residence for 1 yr or more before entering freshwater …