Sr isotopic ratios of 39 glass and microcrystalline basalt samples along the Juan de Fuca Ridge and 1 glass sample from Brown Bear Seamount are at the lower end of the range for normal mid-oceanic ridge basalt (MORB); the average 87Sr/86Sr ratio is 0.70249¿0.00014 (2-&sgr;). Although subtle variations exist along strike of th ridge, the Sr isotope data do not show systematic variation relative to th proposed Cobb Hotspot. The isotopic data are inconsistent with an enriched mantle-plume origin for the Cobb-Eickelberg Seamount chain, as has been proposed for Iceland, the Azores, and the Galapagos spreading center. Sr isotopic ratios of samples collected north and south of the Cobb offset are identical, although minor element ratios indicate that these regions have chemically distinct mantle sources. These distinct mantle sources may not have been separated long enough to develop isotopic differences.