Marine bivalves, especially the Pacific oyster Crassostrea gigas, are economically important in French aquaculture. Nevertheless, the ecology and the physiology of this bivalve are not fully understood; consequently, control of its growth and reproduction both in the field and in hatcheries still relies on empirical factors. The investigation of soft tissues in marine molluscs, especially in marine bivalves, classically relies on destructive methods, since a hermetic shell protects the animal. Non invasive characterization of gonad maturation and determination of the sex of Pacific oysters by Magnetic Resonance Imaging MRI was already successfully tested (Davenel et al., 2006; Pouvreau et al., 2006). MR imaging is the most appropriate technique for quantifying the growth of somatic and gonadic tissues and to determine sex. However, this technique is too costly for field studies or to be applied routinely in hatcheries. In this context, we have tested the ability of low Nuclear Magnetic Resonance relaxometry, a very less expensive technique, to obtain useful phenotypic parameters to control the physiological state of oysters. NMR measurements were performed with an OXFORD MQA 6005 spectrometer operating to 0,12T. (5MHz) and equipped with a 5 cm probe diameter which allowed to investigate oysters in their first year of maturity (< 45 g total weight). NMR measurements were carried out at three different periods (2007 May 7th, June 4th and June 29th) on 60 oysters which were then dissected to measure oyster-shell internal cavity volume, flesh dry weight and to determine sex and gonad development. The NMR results showed that it was possible to determine oyster-shell internal cavity volume but also flesh dry weight in less than one minute with a very high determination coefficient R2 (respectively 0.95 and 0.94). Results showed also that it was possible to identify sex and gonad development, with a rate of success of 83% and 75%, respectively. For the oysters with dry weight higher than 0.7g, the rate of success to identify sex was 100%. Further works are required to design NMR probe well adapted to bigger oysters and to improve sex discrimination and prediction of gonad development with larger collections.