The fine structure of metamorphosis in the athecate hydroid, Hydractinia echinata, is described. Metamorphosis was induced in two ways: by exposing planulae to appropriate microflora and by temporary exposure to an ionic imbalance. Larvae induced to metamorphose by ionic imbalance differed considerably in behavior, but little in fine structure, from those induced by microflora. Metamorphosis was initiated by the discharge of nematocysts and loss of neurosensory cells, followed by secretion of mucus from gland cells and dense-staining granules from supportive cells. Associated with these events is a severe folding of the larva and permanent adherence of the larva to the substratum by its anterior end. Following movement of remaining gland cells across the mesoglea into the endoderm-and movement of interstitial cells into the ectoderm, the metamorphosing planula extends tentacle rudiments and stolonal buds, eventually elongating into a primary polyp.