Very little is known about the potential effects of endophytic infections on the host in algae, especially in terms of fitness. In this study, we report a first attempt at quantifying the effects of the endophytes Endophyton ramosum (Chlorophyta) and Pleurocapsa sp. (Cyanophyta) on the reproductive potential of their host Mazzaella laminarioides (Rhodophyta). Density of reproductive structures and spore germination rates was compared between infected and non-infected fronds. It was found that differences in density of reproductive structures between infected and non-infected fronds were not significant, which suggests that the level of maturity of the host is unaltered by the endophytes. On the other hand, carpospore germination rates significantly decreased when produced by fronds infected by the cyanobacterium Pleurocapsa sp. Thus, in the general context of host fitness, we discuss the fact that only some of the components of the reproductive output of M. laminarioides are negatively affected by the endophytes.