Farmed kelp gametophytes were previously observed to be living endophytically in filamentous red algae. The interactions of two farmed kelp species and six red algae were examined in laboratory culture. Undaria pinnatifida (Harvey) Suringar and Laminaria religiosa Miyabe demonstrated the differing abilities of zoospores to become endophytic in four host red algae and neither kelp became endophytic in two non-filamentous red algae. There was a strong seasonal component regarding infectiousness that is associated with the changes in fron erosion in U. pinnatifida from April to June. At the same time, L. religiosa showed no significant changes in frond erosion, and there were no apparent changes in infection levels in the two species they were able infect. This study indicated clear differences between two keip species with regard to their symbiotic relationship to red algae in terms of host specificity and preference of kelp gametophytes.