A two-year field study of the population of Macrobrachium olfersi from the Guaecá River, São Sebastião, Brazil, revealed a predominance of adult females over adult males, which is expressed by an overall sex ratio of 4.3:1. Relationship between sex-ratio is a function of body size and is related to differential growth with males attaining larger body size than females. Ovigerous females were present in every monthly collection, indicating that reproductive activity was continuous. Periods of higher reproductive activity were observed during the warmer and rainy months. The smallest ovigerous female sampled was 28.1 mm of total body length. Mean fecundity was 1,227 eggs per female (mean total length: 39.9 mm) with a positive correlation between fecundity and female body size. Mean egg sizes (shortest and longest axes) were 0.449 and 0.579 mm at early embryonic development and 0.489 × 0.648 mm at late embryonic development. Based on color and size of ovaries, five distinct developmental stages were recognized. Embryonic development of incubated eggs was usually accompanied by concomitant ovarian maturation, indicating consecutive spawnings during reproductive life span. Fully developed spermatozoa were recognized in histological preparations in males as small as 15.3 mm of body length while previtellogenesis was detected in a female of 26.5 mm of body length.