We investigated seasonal development of the goldspotted oak borer, Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), and physical conditions of the phloem within a preferred host species, coast live oak, Quercus agrifolia Née. We sampled infested trees on a monthly basis at two sites in southern California throughout 2011. Measurements of an exposed portion of the head capsule, the peristoma, indicated that there were four larval instars. Pupae and teneral or mature adults were found within trees from April through August. Adults were captured in flight between May and mid-October, with peak flight from July through August. Within-tree A. auroguttatus prepupal mortality did not differ between sites and increased significantly from 3 ± 3% in January to 36 ± 9% in March (mean ± SE). Prepupae were present in trees throughout most of the year, which made it difficult to determine generation time; it was likely 1 yr for the majority of individuals, and possibly longer or shorter than 1 yr for others. Seasonal A. auroguttatus development, according to within-tree development and adult trap catch, was apparently 2 mo ahead at one site, which had a greater past and current level of A. auroguttatus infestation compared with the other. There was also evidence at the more severely infested site that within-tree A. auroguttatus population densities were positively related to proportion of dying phloem tissue. This suggested that the level of current infestation affected host tree condition, or that dying tissue was more suitable for larval development.