Inhibins and activins are dimeric peptide hormones that play an integral role in the intraovarian regulation of folliculogenesis. The domestic hen, with its well-defined follicular hierarchy, provides a unique model in which to study the role of these hormones in follicular development. In the present study, the complete coding sequence and deduced amino acid sequence for the chicken inhibin/activin β_B subunit has been determined from cDNA clones isolated from a chicken ovarian granulosa cell library. This β_B-subunit cDNA predicts a precursor protein of 392 amino acids containing the mature C-terminal 115 amino acid β_B subunit. When compared to the β_B subunit isolated from a variety of species, the chicken cDNA clone showed high nucleotide identity in the full-length coding region (>70%) and in the mature coding region (>80%). In addition, the deduced amino acid sequence of chicken β_B subunit showed greater than 95% identity compared to other species in the mature peptide region. Expression of the β_B-subunit mRNA was detected by reverse transcription-polymerase chain reaction in both gonadal and extragonadal tissues. Northern blot analysis detected expression in the gonadal tissues only, specifically in the granulosa tissue from the F3-F5 follicles, small yellow follicles (SYF), large white follicles, and immature and mature rooster testes. A major transcript of approximately 4.1 kilobases (kb) and three minor transcripts of approximately 8.4 kb, 6.5 kb, and 1.7 kb were detected in the SYF granulosa samples. To examine the expression pattern of the β_B subunit around the stage of follicle selection, the SYF granulosa was subdivided into two groups: 6–8 mm and 9–12 mm. Quantification of RNA expression (n = 3) showed that expression of the β_B subunit was maximal in the 6–8 mm SYF. Activin B, as well as other intraovarian signals, may regulate early follicle selection and/or development in the chicken.