Mineral phase analysis was performed, using X-ray diffractometry (XRD), for marine-organism shell/skeleton samples of 146–148 extant species of the following 10 phyla (18 classes) collected in Japan: Rhodophyta (Florideophyceae), Foraminifera (Globothalamea and Tubothalamea), Porifera (Hexactinellida), Cnidaria (Anthozoa and Hydrozoa), Bryozoa (Gymnolaemata), Brachiopoda (Lingulata and Rhynchonellata), Mollusca (Bivalvia, Cephalopoda, Gastropoda and Polyplacophora), Annelida (Polychaeta), Arthropoda (Cirripedia), and Echinodermata (Asteroidea, Crinoidea and Echinoidea). Some of the species were analyzed for each specific part of their shells/skeletons. Almost all the samples exhibited any of calcite, aragonite or their mixed phase, predominantly depending on their taxonomy and shell/skeletal structures. For samples containing significant amounts of calcite, the MgCO3wt % of calcite has been determined from their XRD data, which ranges from ∼0 to ∼15 wt % and indicates clear inter-taxonomic differences. Low MgCO3values (∼0–4 wt %) are observed for Rhynchonellata, Bivalvia, Gastropoda and Cirripedia; intermediate values (∼4–8 wt %) for Cephalopoda; high values (∼8–15 wt %) for Florideophyceae, Globothalamea, Tubothalamea, Polychaeta, Asteroidea and Crinoidea; low-to-high values for Gymnolaemata; intermediate-to-high values for Anthozoa and Echinoidea. These MgCO3data show broad trends mostly consistent with general phylogenetic evolution (i.e. very similar patterns for each phylogenetic group). Distinct within-individual variability of the MgCO3content is found for regular Echinoidea species (i.e. their teeth and spines have lower MgCO3values than the other skeletal parts). Correlation of the MgCO3content with seawater temperature is also examined/discussed for most of the above calcite-containing classes. In order to interpret our XRD-based observations of various marine-species shells/skeletons, detailed discussions are presented by comparing with previous studies and also by using knowledge of taxonomy, shell/skeletal structures, habitats, living modes and so on. The comprehensive dataset and discussions will provide useful implications for biomineralization studies.