A winter collection of the intertidal, red algal crust, Hildenbrandia rubra (Sommerfelt) Meneghini, was examined for tolerance of photosynthesis to extremes of temperature, salinity and desiccation. Pulse amplitude modulation fluorometry of chlorophyll a fluorescence was used to determine various photosynthetic parameters including effective quantum yield (Φ_PSII) and relative electron transport rate (rETR). Thalli from the same collection were successively exposed to freezing, desiccation, hyposalinity and high temperature. Thus thalli over 13 days experienced temperature fluctuations from −17 to 27°C, from fully hydrated to extremely desiccated, from full seawater to 4 psu and back, all without any apparent long-term effects to the photosynthetic apparatus. There was no significant difference in Φ_PSII between thalli at the beginning and end of the experiments. Algae showed a significant acclimation to low light in that immediately after collection thalli had optimum light intensity, I_k, in terms of rETR at 50 μmol photons m⁻² s⁻¹, whereas 24 h later maximal rETR was at about 20 μmol photons m⁻² s⁻¹. Hildenbrandia rubra is the most physiologically tolerant of any tested seaweed, and this helps explain its wide geographic and ecological range.