MR imaging is the most appropriate non-invasive technique for quantifying the growth of somatic and gonad tissues and to determine sex in the Pacific oyster, Crassostrea gigas. However, this technique is too costly for field studies where oysters are used as bioindicators of environmental quality or to be applied routinely in hatcheries. We have tested the ability of low Nuclear Magnetic Resonance relaxometry, a much less expensive technique, to obtain phenotype parameters that can be used to monitor the physiological state of oysters. NMR measurements were carried out at three different periods using a low field spectrometer equipped with a 50 mm diameter probe to investigate 60 oysters in their first year of maturity, which were then dissected to measure internal shell cavity volume and dry flesh weight and to determine sex and gonad development. The NMR results showed that it was possible to determine both internal shell cavity volume and dry flesh weight in less than one minute with very high determination R-2 coefficients (0.95 and 0.94, respectively). The results showed also that it was possible to identify sex and gonad development, with success rates of 93% and 83%, respectively. For oysters with dry weight above 0.7 g, the success rate in identifying sex was 100%. Further studies are required to design an NMR probe that is appropriate for larger oysters and to improve sex discrimination and prediction of gonad development with larger study groups. (c) 2009 Elsevier B.V. All rights reserved.