The two kelp species Alaria esculenta and Laminaria digitata are co-occurring in the shallow sublittoral at a depth between 0 m and 8-10 m at our Arctic study site in Kongsfjorden, western Spitsbergen but they also co-occur at their southern distribution boundary in Brittany, France. The physical driving forces for their vertical and geographical distribution pattern are partially known but the effect of abiotic factors on interspecific competition of these con-specifics has never been investigated. The latter might become more and more important, especially under future global warming scenarios. In our study, we thus investigated the interaction of early recruitment stages (spores, gametophytes, juvenile sporophytes) between A. esculenta and L. digitata under varying temperature conditions on different spatial (µm to cm) and temporal scales (days to weeks), comparing responses of clonal and freshly derived gametophytes in laboratory experiments, either in single culture controls or diverse co-cultivation set-ups. Germination and development into sporophytes of both species were positively stimulated by a temperature increase from 5°C to 10°C. In 15°C, however, the development of A. esculenta was strongly retarded and although some eggs were released from oogonia, no sporophytes developed. In general, sporophyte development and growth was much faster in A. esculenta than in L. digitata leading to out-competition of the latter species in long-term experiments at 5°C. During gametogenesis transient interspecific competition effects occurred, but always in favour of A. esculenta at xy °C. The susceptibility towards both, species interaction and temperature, was species-specific and different developmental stages were affected differentially. All observed interaction effects between the two kelp species may be interpreted as a consequence of resource competition and less likely as consequence of allelochemical interaction.