Summer mortality of the Pacific oyster Crassostrea gigas is the result of a complex interaction between oysters, their environment and their pathogens. The physiological status of an oyster, especially its reproductive status, is suspected to play a significant role in the outcome of this interaction. As genetic variability exists for susceptibility to summer mortality, resistant (R) and susceptible (S) oyster lines were produced using a divergent selection scheme. The present paper reports a histological study on gonad area, which is representative of reproductive effort, in randomly chosen five R and five S oyster lines. The R lines showed a significantly lower gonad area than the S lines (P < 0.001), with an estimated mean difference of 12.5%, whereas, taken together, R and S lines showed a similar distribution of gametogenic stages when sampled. Considering the lines separately, the significant difference in gonad area went up to 24% between R and S lines. The present data confirm and strengthen the negative correlation between reproductive effort and resistance to summer mortality observed in previous studies. Summer mortality of C. gigas in France could, therefore, be partly due to a physiological disorder and metabolic disturbance in oysters associated with their reproductive effort. This does not, however, imply a direct link between the cost of reproduction and mortality because other causal factors, such as pathogenic agents, could be the primary causal factors.