This study examined the parasitic interaction between Andricus californicus, a species of cynipid wasp, and Quercus lobata, the valley oak, at Jasper Ridge Biological Preserve in San Mateo County, CA. A. californicus oviposits its eggs in the tissue of valley oaks, resulting in the formation of large, baseball-shaped detachable stem galls on the trunk and branches of the tree. I sampled 54 valley oak trees throughout the month of May, 1998. For each tree, I recorded the number of old and new galls (easily distinguishable on the basis of color), tree height and circumference, and habitat. The data differed significantly from a random Poisson distirbution. No correlation was found between height or circumference and number of new galls per tree. A chi-square test determined that proportions of parasitized trees did not vary depending on habitat. An ANOVA found that the mean number of galls per tree did not differ significantly between habitats. Using a chi-square, I found that the presence of new galls varies dependently [sic] on the presence of old galls. This suggests a third factor that determines presence of both new and old galls. This factor could be a smell or visual cue making the tree desirable for parasitism season after season, or the fixed ability of the tree to react to a threshold level of wasp activity. The third factor could also be old galls themselves. Possible sources of experimental error include difficulty of sampling and confusion of galls with those of other species.