Microconstituents were measured in otoliths of juvenile northern bluefin tuna (Thunnus thynnus) from the Mediterranean Sea and western Pacific Ocean. Whole otoliths were analyzed by inductively coupled plasma mass spectrometry. Multivariate statistical analysis showed clear separation between the two groups. Na, Mg, Mn, and Zn were most useful in differentiating Mediterranean and Pacific samples. Findings justify further use of otolith microconstituents to investigate stock structure questions concerning Atlantic bluefin tuna. (C) 1998 Elsevier Science B.V. All rights reserved.