The present study investigated seasonal changes in gonad development and proximate compositions of the mangrove snail, Cerithidea obtusa in the mangrove forest of Ca Mau province, Mekong Delta of Vietnam. Our findings showed that gametogenesis of C. obtusa occurred year around, however, the major spawning period lasted from September to November. The highest spawning synchrony was observed in November with 43% collected individuals in spawning stage. The results also showed that in mangrove snails, proteins are the major component (~60%) followed by carbohydrates (~25%) and then lipids (~5%). Proteins could be the major energy resource to support growth and reproductive cycle of this species. Results from this study provide detail information on the biology of mangrove snails and it could be useful for resource management, bioconservation and sustainable aquaculture in mangrove forests of Vietnam.