Clonal origins of Rohon-Beard neurons in Xenopus were determined quantitatively by injecting horseradish peroxidase into individual blastomeres at the 16-cell stage and later counting labeled and unlabeled Rohon-Beard neurons. Two different patterns of cleavage were selected. In pattern X, all Rohon-Beard neurons originated from three blastomeres (V1.1, V1.2, and V2.2) on each side; in pattern Y, all Rohon Beard neurons originated from two blastomeres (V1.2 and V2.2) on each side. Counts of Rohon-Beard neurons at larval stages 32 to 34 showed that 96 to 100% (mean 99%) originated from blastomeres on the same side; of these, 68 to 90% (mean 75%) descended from V1.2, 20 to 31% (mean 24%) descended from V2.2, 0 to 7% descended from V1.1. The significance of the regionally restricted origin of Rohon-Beard neurons is discussed.