Sex ratio and morphological traits of a very abundant anchovy Anchoa januaria were described in a tropical bay in south-eastern Brazil. The aim was to test the hypothesis that sexual dimorphism occurs due to the different reproductive roles of the sexes. A fish sampling programme was carried out between September 1998 and August 1999 at six sites: four sandy beaches and two lower-river sites. Population structure at river sites comprised adults only, ranging from 60 to 80 mm total length (L T ), while at sandy beaches both juveniles and adults were found, ranging from 32 to 80 mm L T . Well-balanced 'spawning school' at river sites during reproduction were detected, while female-dominated schools occurred in the bay feeding areas. Males had relatively longer pectoral fins, slightly larger hearts and more somatic mass than females. Females outnumbered males at sizes >67 mm L T and had significantly longer intestines and heavier livers than males. The largest size reached by females was probably related to a higher growth rate as they have a larger intestinal absorbing area for nutrients. The prediction of higher energetic investment in reproduction by females that should have larger organs associated with food acquisition and processing to produce energy-rich eggs was confirmed for A. januaria in Sepetiba Bay.