An odontogenic myxoma of a maxilla has been examined. Histochemistry of the mucosubstance indicated that hyaluronic acid and chondroitin sulphate were present. On ultrastructural examination many of the cells in the myxomatous tissue were seen with prominent rough endoplasmic reticulum, suggesting a secretory function, and possibly the myxomatous ground substance was produced by these cells. Cells containing collagen fibrils were found. Epithelial islands with intercellular spaces producing an arrangement similar to the stellate reticulum of the enamel organ were found, and possibly had developed from odontogenic epithelial rests. These islands were surrounded by a clear zone, outside which were cells with an increased prominence of adenosine-triphosphatase, nucleoside-diphosphatase, thiamine-pyrophosphatase, and often arylamidase reaction products. This probably represents a reaction of the mesodermal tissue to the epithelial islands. Possibly the epithelium exerts an inductive effect on the mesodermal tissue, and the myxomatous appearance may be the result of an aberrant development of mesodermal cells into ;myxoblasts' which secrete the myxomatous ground substance.