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The understanding of infectious diseases of algae has improved significantly in recent years, particularly in the area of recognition and signaling, key processes that determine the success or failure of host invasion by the pathogen. Ecological studies have also contributed to better understanding the role of diseases in wild stands of the affected hosts. An aspect that has received only limited attention is the effect of the infections on host fitness, and in this context, we report a first attempt to quantifying the effects of Pleurocapsa sp. (Cyanophyta) on the reproductive potential of its host Mazzaella laminarioides (Rhodophyta). Infections by Pleurocapsa trigger the development of tumors that can result in major changes in frond morphology and texture. Two populations of the host were considered in the study. Our results indicate that infections do not cause a significant effect on the density or quality of the reproductive structures (i.e. cystocarps and tetrasporangia). However, the number of spores, settlement rates, germination success and offspring survival, were all affected negatively by the endophytic infections. The reported information and field-collected data, strengthen the notion that pathogens of algae may exert strong effects on their hosts at several levels, including reproduction. These effects can vary from host death during infections by highly pathogenic organisms to more subtle effects like those observed in the studied pathosystem. Infections by less aggressive pathogens, however, still may determine important effects at the population level by inducing differential mortality and reproductive success in infected individuals. [ABSTRACT FROM AUTHOR]