This study uses data from field introduction experiments with Gammarus spp. conducted by H. B. N. Hynes in the Isle of Man in the period 1949-1969, together with data from re-sampling programmes by the present authors in 1974/5, 1988 and 1995, to examine the interaction between the native freshwater amphipod G. duebeni and the introduced G. pulex. Although Hynes' deliberate introductions of G. pulex into streams in the Isle of Man at first seemed to give equivocal results, their examination in the light of subsequent evidence indicates that G. duebeni is, under some as yet unknown condition(s), sometimes able to withstand the impact of invasions by its competitive rival G. pulex. The presentation of Hynes' data is thus a belated but valuable contribution to knowledge in this field, although we must stress that such experiments involving transplantations of species should not be carried out owing to the ecological damage that can follow. In addition, we discuss our discovery of a truly exotic amphipod in the Isle of Man, the N. American Crangonyx pseudogracilis.