We estimated the range expansion of Barred Owls (Strix varia) in California and the potential negative effects this species may have on Spotted Owls (Strix occidentalis). Barred Owl range expansion has been rapid over the past two decades. A total of 61 Barred Owls has been detected in 12 different California counties. Barred Owls were first sighted in California in 1981 in Del Norte and Trinity counties. They now have been detected as far south as Sonoma County in western California and Yuba County in the Sierra Nevada. The ratio of new Barred Owl sites found per new Spotted Owl territory has increased from one per 50 Spotted Owl territories in the 1980s to one per 10 to 20 new Spotted Owl territories in the mid 1990s. This suggests that the Barred Owl population in California is increasing. In addition, seven Barred Owl x Spotted Owl hybrids have been reported in California. Because of the potential for hybridization, competition for food and habitat, and predation, it appears that the Barred Owl could influence Spotted Owl populations negatively.