Marine County, California, is the southern limit of the range of the Northern Spotted Owl (Strix occidentalis caurina), listed as threatened by the U.S. Fish and Wildlife Service. The density of the Marine population of the Northern Spotted Owl is unusually high, the population breeds in unique habitat associations, and it is genetically isolated from other Spotted Owl populations. Unlike elsewhere in the Northern Spotted Owl’s range, habitat loss to logging is not an issue in Marine County. The Barred Owl (Strix varia) has been detected in Marine County only since 2002 and may pose a threat to the Northern Spotted Owl through competition and/or interbreeding. We amassed information on the distribution and abundance of the Barred Owl in Marine County via published literature, by consulting local birders, and primarily through data we obtained during our monitoring of the Northern Spotted Owl in Marine County. Monitoring, continuous since 1996, provides an opportunity for an evaluation of the effect of the Barred Owl invasion on the Northern Spotted Owl there. We estimate the county’s current population of the Barred Owl at four to seven individuals, including one territorial pair and a single territorial male. We documented two nestings, with four young fledged. Two pairs of the Northern Spotted Owl have been displaced from territories. These results are of concern for an otherwise stable population of the Northern Spotted Owl.