We isolated a novel gene by a differential-display RT-PCR method comparing basal disk tissue and peduncle tissue in a species of Hydra, Pelmatohydra robusta, and we referred to it as anklet. The putative anklet product has a signal sequence in its N-terminus, and it has one MAC/PF domain and one EGF domain. In normal hydra, the expression of anklet was restricted in the periphery of the basal disk and the lowest region of the peduncle. In foot-regenerating animals, anklet was first expressed in the newly differentiated basal disk gland cells at the regenerating basal end, and then expression became restricted at the periphery of the regenerated basal disk and in the lowest region of the peduncle. This spatially specific expression pattern suggested that the product of the anklet gene plays a role in basal disk formation. We therefore examined the role played by the protein product of the anklet gene by suppressing the transcription level of anklet using an RNA-mediated interference (RNAi) method. Suppression of the level of expression of the anklet gene led to a decrease in basal disk size in normal hydra, and to a delay in basal disk regeneration in foot-amputated animals. These results suggested that anklet is involved in the formation and maintenance of the basal disk in hydra.