ABSTRACT. Distribution and abundance of mullcts (Ostcichthycs, Mugilidae) in the Sepetiba Bay, Rio de Janeiro, Brazil. Variations in the fish populations of mulIets were assessed aiming to detect trends in abundances during their early life cycle and relationship with environmental parameters. Fish were sampled between August 1994 and July 1997 usinga beach scine and environmental parameters ofwater temperature, salinity and transparency were taken at each sampling. Five sites were systematicalIy sampled, two ofthem in the inner Bay, and three in the outer Bay near to thesea limit. Neste trabalho abordam-se aspectos de distribuição e abundância de jovens da família Mugilidae na Baia de Sepetiba, visando determinar a influência das condições ambientais da Baía nos seus ciclos de vida, e contribuir para o conhecimento de mecanismos de distribuição espacial e temporal das espécies no ecossistema costeiro.