A seasonal development study of the cerambycid beetles Phoracantha recurva Newman and Phoracantha semipunctata (Fabricius) (Coleoptera: Cerambycidae) was conducted under field conditions to look for differences in the developmental parameters of these congeneric species in southern California. Neonate larvae were introduced into eucalyptus logs in February, May, July, and October, and the infested logs were held in field cages at two locations: an inland desert site and a more temperate coastal site. Development times from neonate larva to emerged adult and percent emergence were recorded for beetles from all log cohorts at both sites. Under southern California climatic conditions, P. semipunctata seems to complete only one generation per year. Adult P. semipunctata emerged from infested logs from June to October. In contrast, P. recurva seems to be able to complete one generation and begin a second generation within the same year. Adult P. recurva emerged from infested logs from February to October. Across all treatments, a greater percentage of P. recurva (70 ± 2.5%) completed development in host logs than P. semipunctata (64 ± 1.5%). The majority of P. recurva (74.9%) and P. semipunctata (99.4%) emerged during June, July, and August. Host quality was correlated with beetle size. Overall, the differences in the developmental parameters of the two species may be contributing to the replacement of P. semipunctata by P. recurva in their shared habitat niche in southern California.