Previous studies comparing Crassostrea gigas selected lines with different susceptibility to summer mortality concluded that the higher the reproductive effort the lower the survival. But theywere lacking individual followup of this reproductive effort which would be of great interest to help us to understand the physiology of the oysters facing summermortalities. Themost frequently usedmethod for assessing the volume of the gonad is to measure the area it occupies on histological sections. Gonad measurement by magnetic resonance imaging, in contrast, is non-destructive and therefore makes it possible to assess individual evolution as well as to compare individuals. The present study compared the values of gonad volume by thismethodwith the values of gonad surface obtained by the histological method. The two methods were used successively to assess the volume of the gonads of the same individual oysters at different maturation stages. There is a very highly significant linear relation between the results of these two methods. The estimated uncertainty about the volume value is 29%. Causes of this uncertainty are considered and discussed.