The blue crab, Callinectes sapidus Rathbun, 1896, was studied because of its high dollar value to Florida's commercial and recreational fisheries. The purpose of this study was to describe the structure of the ovary and oogenesis in the blue crab and the morphological changes in the female reproductive developmental stages over time. Histological techniques for high-resolution light microscopy were used to determine sexual maturity of female blue crabs. The ovarian morphology, oogenesis, and changes through the annual reproductive cycle of blue crabs in Tampa Bay were investigated for a period of two years, from January 2005 to January 2007. Ovarian structure was assessed by analyzing histological sections embedded in plastic epoxy resin, which provided a higher resolution than any other embedding material previously used in research on blue crab reproduction. Qualitative analyses of female gonads were made by describing the structure of the oocytes and determining the developmental stage of the oocytes from oogonia to full-grown oocytes. This study developed and introduced a new reproductive staging criteria for the species. Morphological characteristics of ovarian tissues and oocytes were determined to develop a classification for oocyte maturation stages. Morphological changes in the oocytes are well defined, and these were used to develop the staging schema. In this study, it was found that carapace width is not a good indicator of maturity or developmental stage. Examination of the annual reproductive cycle indicates that late secondary growth occurs from July to March, and gravid crabs were found during November and December. Histological examination of ovarian tissue is essential for determining maturity in female blue crabs. By observing ovarian characteristics and by establishing the length of secondary growth during oogenesis in blue crabs of Tampa Bay, a more thorough understanding of the cyclic reproductive aspects of this species was obtained and specifically that animals at a carapace width between 100 mm and 125 mm may have mature oocytes, yet external features may not indicate that they are mature.