The effect of temperature, air exposure, salinity and light have been tested for three species of Gelidium on the basis of photosynthetic and growth responses in order to assess the zonation patterns observed on the Atlantic coasts of northern Spain. The vertical distribution of three Gelidium species is described: G. sesquipedale reaches its limit in the low intertidal zone, whereas both G. latifolium and G. pulchellum extend from the mid- to the lower intertidal zone. These differences in location on the shore gradient are in accordance with the physiological responses to the factors tested. Gelidium sesquipedale has the lowest saturated net photosynthesis (Pmax) and saturating (Ik) Photosynthetic Photon Flux Density (PPFD) and is the less tolerant to changes in temperature and to dessication. Exposure to air causing water losses above 50% (wet weight) produced negative net photosynthesis values in G. sesquipedale, Gelidium latifolium showed broader tolerance ranges to the factors mentioned above, although exposure to long periods of dessication caused significant reductions in the photosynthetic response. Gelidium pulchellum, which has the highest vertical position in the shore, was the most tolerant to temperature and dessication. The response was similar when growth, instead of net photosynthesis, was tested against various combinations of temperature and salinity. These differences in net photosynthesis and growth are consistent with the observed zonation pattern. Although other causes can be invoked. it is concluded that tolerance ranges are decisive in determining the vertical distribution of these three species.