The effect of diet on the life-span of female Nasonia vitripennis (Walk.) has been investigated. Life-tables were constructed for these insects fed on dipteran puparia and on 10% sucrose; Further experiments were performed involving the transfer of females from the sucrose diet to puparia at different ages. The converse experiment was also carried out. From this work it was shown that the insect is capable of a greatly extended life if fed sucrose for all, or part of its life-span. However, transfer from puparia to sucrose after 15 days allows an increased survival for only a small percentage of the population. The results are discussed with regard for the age at transfer and to the reproductive physiology of the insect.