This article aimed to study the effects of phlorotannins from the brown algae C. costata on α-NaGalase produced by duodenal adenocarcinoma HuTu 80 and melanoma SK-MEL-28 cell lines, namely, the suppression or increase in the activity of α-NaGalase in cancer cells, as well as the direct inhibition or activation of isolated enzymes. This evidence concerns the gene NAGA and cancer.