Laminaria sinclairii (Harv.) Farl., And., & Eat. occurs from central British Columbia to southern California. L. longipes Bory occurs from the Kurile Islands throughout the Aleutian Islands to southeast Alaska. Detailed distribution records are presented. Both species are distinguished from other species of Laminaria in having multiple stipes from an expanded, rhizome-like holdfast. The two species are very similar in appearance. Most authors have distinguished them on the basis of internal stipe anatomy; L. sinclairii has mucilage ducts in the stipe, whereas L. longipes does not. Ecological studies were carried out on both species in the field and in the laboratory. The results show the species also differ in several other points, including length of stipes, width of blades, winter loss of blades, morphology of gametophytes, and habitat. The evidence confirms that they should be retained as two separate species.