The axial position of interstitial-cell (i-cell) differentiation into nematocytes inHydra was studied. Nests of developing nematoblasts of three types of nematocytes were distributed in a non-uniform manner along the body column. Stenotele nematoblasts were distributed in a gradient with a maximum in the peduncle. Desmoneme and atrichous isorhiza nematoblasts were found predominantly in the upper half of the body region. These results suggest that the type of nematocyte differentiation an i-cell undergoes is influenced by the axial position of the i-cell. Because the assayed stage of nematocyte differentiation occurred 6-7 days after beginning of differentiation, the axial position of the anticedent i-cell at the time of commitment was determined by correcting for tissue displacement.