During the summer of 1962, as a preparatory investigation toward a taxonomic monograph of the genus Rorippa (Cruciferae) for North America, I studied one of the extremely variable species, R. islandica (Oeder ex Murray) Borgas, marsh yellow cress, in the Douglas Lake region of northern Michigan. In our region two varieties are recognized, var. fernaldiana Butters et Abbe and var. hispida (Desvaux) Butters et Abbe. They are morphologically separable on fruit shape and distribution of pubescence on the plants. This study was conducted to reveal the kinds of habitats in which these two varieties of R. islandica grow and to determine with which plant species they most often associate.