Sex ratio and size–gender distribution of Coralliophila meyendorffii and Babelomurex cariniferus collected in the field was studied. While sex ratio did not depart from the expected Fisher 1:1 ratio for dioecious species, the size–frequency distributions showed clearly a sexual size dimorphism with males significantly smaller than females. Laboratory monitoring of penis reduction in both species, combined with an anatomical and histological study of the reproductive system of individuals reducing penis revealed the existence of transitional sexual stages, proving the existence of protandry. Penis monitoring also demonstrated a high plasticity in size at sex change, suggesting a socially controlled sex change. In C. meyendorffii, where penis monitoring followed an experimental design based on the assumption that protandry existed and that presence of females influenced sex change, results strongly supported a social control of sex change. Evidences for protandry in other coralliophilids are summarized and protandry as a widespread sexual strategy in Coralliophilidae is postulated.