The aim of this study was to test the ability of magnetic resonance imaging (MRI) technique to characterize gonad development and to determine the sex of live Pacific oysters through their shells. A preliminary nuclear magnetic resonance (NMR) retaxometry study was conducted to characterize T-1 and T-2 NMR relaxation parameters for the main oyster organs. This showed that T-1-weighted MRI sequences were most appropriate to optimize contrasts between tissues in images. The results showed that gray levels of gonads in images acquired with gradient-echo sequence were variably affected by T-2* weighting effect. However, the ovaries systematically gave a hypersignal in spin-echo T-1-weighted images, and stack histograms of female oysters showed a peak well separated from that of male oysters. An automated method is proposed to quantify the development of oysters and their gonad maturation and to identify their sex. (C) 2006 Elsevier Inc. All rights reserved.