We investigated for early and late blooming walnut cultivars in California whether variation in nut phenology resulted in differences in nutritional quality and whether this, in turn, affected the performance of the codling moth, Cydia pomonella (L.), and the extent of nut damage. 2. Mid-season, during the period of nut growth, nuts from the early cultivars were larger than those from the late cultivars and had higher nitrogen content in both husk and kernel tissue, while kernel phenolic content was significantly lower. No major differences were observed later in the season after nuts from all cultivars had reached their final size. 3. Throughout the season establishment of neonate larvae was highest on nuts from the early cultivars but this was only significantly so at the beginning of the third codling moth generation. During the second codling moth generation (mid season) relative growth rates of third-instar larvae were significantly higher on early than on late cultivars. Nut damage in the field was also significantly greater on early than on late cultivars during generation 2, while no significant differences were obs 4. The data suggest that the variation in codling moth damage among walnut cultivars is related to bloom phenology due to the influence of nut phenology on larval performance.