Calmodulin (CaM) is essential in plants for signal transduction under environmental stress. To determine the function of CaMs in the thermal response of the red alga Gracilaria lemaneiformis, six cams were cloned from three strains, wild type and heat-tolerant cultivar 981 and 07-2. In wild type, w-cam1, w-cam2, and w-cam3 were cloned; in cultivar 981, 981-cam1 and 981-cam2 were obtained; and in ... [Show full abstract] cultivar 07-2, only 072-cam1 was identified. The cam1s in the three strains were identical. They contain 832 nucleotides, including four exons and three introns. The ORFs of w-cam2 and 981-cam2 were the same length, having 450 nucleotides without introns, and they shared 78 % similarity in nucleotides and 87 % in amino acids. In addition, w-cam3, a gene of 809 nucleotides, consisting of three exons and two introns, was cloned from the genomic DNA of wild type. The transcription levels of the cams under heat shock were tested by real-time quantitative PCR, except for w-cam2 and w-cam3, because these two cams could not be amplified effectively by RT-qPCR. Among all the cam1s, 981-cam1 expressed more highly under heat shock. Though w-cam1 and 072-cam1 were down-regulated under heat, the suppression level of 072-cam1 was slightly less than the wild type. For 981-cam2, the expression level was lower than that of 981-cam1 at 28 °C, while at 32 °C, the two cams of 981 had similar expression profiles. These results indicate that the cams may play an important role in G. lemaneiformis and that cam1 responds the most to heat shock.