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The evolution of zooxanthellate endosymbionts in modern coral lineages is relatively well known, originating in the Triassic with the rise of scleractinian corals. It has been suggested that this symbiotic relationship may have evolved before the Triassic, sometime in the Paleozoic. This is a study of proxy evidence for zooxanthellate endosymbionts in a species of rugose coral, Siphonophrentis gigantea, from the Columbus Limestone (Devonian) of Ohio, USA. X-ray Computed Tomographic (XCT) scans were conducted on 10 specimens of Siphonophrentis to visualize internal growth banding, reflected as density differences in skeletal material. Growth bands were measured and examined for changes in growth pattern. Measurements of growth bands showed a range of yearly growth from 3.958 mm to 15.832 mm, with an average of 8.347 mm/year. Only five of the 10 scanned specimens had measurable growth bands preserved. Of these specimens, two showed quite high growth rates, consistent with the presence of a zooxanthellate symbiont. One specimen showed a substantial, short-term disruption in growth, suggestive of a bleaching event, including loss of the zooxanthellae.