Deviation from pentamerism is a rare phenomenon in 5-armed echinoderm species. Qualitative and quantitative data from field observations of 14 populations around the world were used for a biogeographical study of this phenomenon. Four abnormal forms presenting deviation from the classical pentamerism of echinoderms were observed within the brittle-star species Amphipholis squamata (Echinodermata: Ophiuroidea). Although these forms were rare in global populations (0·29%), they can reach important frequencies in some locations. Our results suggest that habitat structure has an influence on the occurence of abnormal forms. Moreover, deviations from pentamerism are not a heritable character but are a consequence of environmental perturbations on the metamorphosis of larvae and/or abnormal regeneration of arms.