17β-Estradiol (E₂) is among the most important sex steroid hormones during ovarian development and vitellogenesis in crustaceans. In this study, an immunofluorescence assay was used to identify the immunopositive distribution and variations in E₂ in the ovary, hepatopancreas, thoracic ganglia, brain ganglion, and the mandibular organ during ovarian development in the swimming crab, was mainly distributed in the follicular cells and the cytoplasm of late-stage oocytes (II-V). Follicular cells stained strongly positive for E₂ at all ovarian stages; the immunoreactivities of E₂ in the oocyte cytoplasm and nucleus trended as "low-high-low", and maximum positive staining appeared during ovarian stage IV and stages II-III, respectively. In the hepatopancreas, E₂-positive staining was mainly distributed in F cells and R nuclei, and E₂ immunoreactivity in the R nucleus tended to decrease from stages III to IV. However, strong positive E₂ immunoreactivity was always detected in F cells at all ovarian stages. E₂ was mainly distributed in neurons, the nerve medulla of thoracic ganglia, and cell nuclei of cerebral ganglion nerve cells during the early stages of ovarian development (I-II). E₂ immunore activity in the nerve medulla of the thoracic ganglion was moderately positive, where as neurons were always weakly positive. Strong positive E₂ staining was always distributed in the nuclei of mandibular gland cells and the cytoplasm around mandibular gland cellnuclei. These results suggest that E₂ is widely distributed in the ovary, hepatopancreas, thoracic ganglion, cerebral ganglion, and mandibular gland of immunoreactivity and ovarian developmental stage. E₂ may act through multiple target organs to regulate ovarian development in