Diverse phenotypic characteristics have evolved in seaweeds to cope with environmental stress, but these traits can vary among populations of the same species especially if these are distributed along environmental gradients. In this study, natural populations of the carrageenophyte Chondracanthus chamissoi and the agarophyte Gelidium lingulatum from a latitudinal gradient along the Chilean coast (between 20 degrees S and 41 degrees S) were compared. We determined physiological and biochemical traits in field and culture samples. Sulfated polysaccharide contents ranged from 15.4 to 52.7% dry weight (DW) in C. chamissoi and from 10.9 to 25.1% DW in G. lingulatum. Carrageenan amounts were higher in gametophytes than tetrasporophytes and were also, depending on life cycle phase, negatively correlated with the geographic variation of temperature, photosynthetically active radiation (PAR), and chlorophyll a (Chl a), whereas agar showed no significant correlation with these variables. The UV-absorbing mycosporine-like amino acids (MAAs) shinorine and palythine in both species ranged from 0.8 to 6.8 mg g(-1) DW and these contents were positively correlated to PAR and Chl a levels at the sampling site. In C. chamissoi variation among populations in their photosynthetic characteristics, pigment concentrations, antioxidant capacity, and MAA contents were persistent after acclimation under common-garden conditions, suggesting ecotypic differentiation in this species. Contrary, G. lingulatum seems to have a more generalist strategy because differences after cultivation were observed only in some photosynthetic parameters and phycobiliprotein concentration. This study confirms that intraspecific differences in phenotypic traits along the same geographic area are strongly dependent on species and life cycle phases.