In echinoid larvae the adult rudiment develops on the left side of the larval body at the eight-armed pluteus stage. Previously we found that a removal of a portion of ecto-mesodermal tissues on the right side of echinoid gastrulae evoked reversal of the left-right location of the adult rudiment in some treated larvae (Aihara and Amemiya, 2000). In the present study we extirpated a portion of tissues on the right side of the larvae at the two-armed pluteus stage and examined effects of the operation on the development of the adult rudiment. Many of the operated larvae formed an extra right adult rudiment, resulting in double adult rudiments. The rest of the operated larvae formed the adult rudiment normally on the left side. No larvae with only one right adult rudiment were found in the experiment, indicating that the formation of the extra adult rudiment on the right side did not affect adult rudiment formation on the left side. In larvae with double adult rudiments, no notable morphological defects in the extra adult rudiment formed on the right side were found, although initiation of the adult rudiment formation was slightly retarded compared to that on the left side. The larvae with double adult rudiments were competent to metamorphose into conjoined juveniles, both the bodies being joined at their aboral sides. The results indicate that the potential to form the adult rudiment is still retained in the right side of the echinoid larvae at the two-armed pluteus stage, suggesting that the fate of the right side with respect to initiation of the formation of the adult rudiment is not committed at this stage.