The unicellular sporophyte phase of the filamentous green alga, Acrosiphonia (Codiolales), is endophytic in the bladed red alga, Mazzaella splendens in southern British Columbia, Canada. Since endophyte maturity coincides with decreased host abundance (winter storms dislodge Mazzaella blades), many endophytes lose their host before zoospore release has occurred. We investigated whether the endophytes can reach reproductive maturity within decaying host tissue (drift blades) or as free-living entities once their host has disappeared. We demonstrate that both occur. Suspension of M. splendens blades, colonized by the endophyte phase of Acrosiphonia, in seawater tanks in winter, resulted in the germination and growth of filamentous Acrosiphonia plants (gametophytes) in spring. Net primary productivity of 132·2 g dry wt/m2/mo was estimated by harvesting total Acrosiphonia biomass after a one-month growing season. An estimate of Acrosiphonia survivorship from zoospore to macroscopic adult gametophyte (0·01–0·89%), is in close approximation with kelp spore survivorship to microscopic gametophyte (0·005–0·1%), but is 102–105 times higher than kelp survivorship from spore to macroscopic adult sporophyte. Differential survivorship in the field and in experimental tanks and among different study sites can be explained by variable mortality factors for spores and germlings and Acrosiphonia's success in colonizing highly disturbed environments.