AbstractYellow starthistle, Centaurea solstitialis L., is a noxious weed that infests more than 7 million ha of rangeland in California. The rust fungus, Puccinia jaceae Otth var. solstitialis, was first released as a classical biological control for yellow starthistle in California in 2003. In 2005, a research program was initiated to monitor the life cycle and spread of P. jaceae solstitialis. The rust was released at two sites representing different climatic zones, the coastal hills and Central Valley, in January 2005 and 2006. Releases resulted in infected plants at both sites in both years. Natural urediniospore (infective spore) reinfection occurred throughout yellow starthistle's growing season at the Central Valley site, but infection did not persist at the coastal hills site. P. jaceae solstitialis spread at least 100 m in 2005 at the Central Valley site but did not spread at the coastal hills site. The results of this study show that the spread of the rust is most concentrated in areas closest to release sites. Teliospores (dormant spores) were produced during plant senescence at both sites in 2005. Our results suggest that P. jaceae solstitialis is likely to establish and spread to new yellow starthistle populations in the Central Valley but not in the coastal hills, near Napa California.