Few animals of marine origin are found living successfully in the high littoral fringe of tidal shores, which thus can be regarded as an extreme environment for such animals. In Iceland, a notable exception is the semiterrestrial Orchestia gammarellus (Pallas, 1766) (Amphipoda: Talitridae). The discovery of the harpacticoid Itunella muelleri (Gagern, 1922) in cultures of O. gammarellus prompted us to investigate the harpacticoid further. Qualitative samples were obtained from a number of locations around Iceland, as well as from Norway and Scotland. The samples were collected from habitats where O. gammarellus was present or at similar tidal levels where this species was absent. Two of the sampling sites in Iceland were under the influence of warm freshwater springs. Itunella muelleri was generally common in the upper part of the littoral fringe of rocky shores in Iceland and was found in similar habitats in Norway and Scotland. To estimate the vertical distribution of I. muelleri, a transect was taken along the shore in Hvassahraun, southwestern Iceland. On the transect, I. muelleri was restricted to the uppermost stations, being the only identifiable harpacticoid found at the highest station, well above the level of highest predicted tides. Animals living successfully in the high littoral fringe must be able to tolerate a degree of desiccation and a wide range of salinities and temperatures. Laboratory experiments showed that I. muelleri thrived in high humidity out of water and also in 8 ppt and 32 ppt seawater, at both 7° and 20°C. The animals survived for months in fresh water, but they only fed sparsely and did not reproduce. The tolerance of I. muelleri to extreme environmental conditions could make this species useful for experimental purposes or as food for fish larvae in aquaculture.