In the plastics industry, bisphenol S (BPS) replaces sisphenol A reported to be an oestrogen mimetic endocrine disruptor damaging oocyte meiosis and maturation (Machtinger 2014 Reprod. Biomed. Online 29, 404). Studies on fish and rodents reported that BPS affects reproduction similarly to BPA (Uzumcu 2007 Reprod. Toxicol. 23, 337; Giulivo 2016 Environ. Res. 151, 251; Ullah 2016 Chemosphere 152, 353). Bisphenol S is detected in human urine at nanomolar concentrations (Liao 2012 Environ. Sci. Technol 46, 6860) and in some laboratory supplies (tips and tubes; unpublished data). Therefore, in this study, we assessed effects of BPS at low doses during in vitro maturation (IVM) on oocyte developmental competence in ewes. Cumulus-oocyte complexes (COC) collected from ovine follicles >2 mm underwent 24-h IVM, in the absence or presence of BPS at 1, 10, and 100 nM and 1 and 10 µM (Sigma Chemical Co.). Nuclear oocyte maturation rate was evaluated by MII oocyte count after chromatin Hoechst staining [n = 3 replicates (R), 1159 oocytes]. At 6 h of IVM, BPS effects on mRNA expression of oestrogen (E2) and progesterone (P4) receptors in cumulus cells (CC) were assessed by real-time quantitative PCR. After 24 h of IVM, The effect of BPS on P4 level was assessed in spent medium by enzyme-linked immunosorbent assay (n = 6 R, 40 COC/condition). Transcript expression level and P4 concentration were analysed using nonparametric one-way ANOVA, with Tukey post hoc test (Rcmdr, R version 3.5.3). After 24 h of IVM, matured COC underwent IVF and in vitro culture (IVC) for 7 days. Cleavage and blastocyst rates were assessed on Days 2 and 7, respectively, after IVF (8 experiments, 300 COC/condition). Data were analysed using logistic regression and linear model (R version 3.5.3). Our results showed a decreased oocyte maturation rate with 10 µM BPS (76.6%, n = 171; P = 0.0008) compared with control (88%, n = 152), with no effect on cell viability. The concentration of P4 decreased with 1 µM BPS (0.02 ng mL−1 per COC) compared with control (0.034 ng mL−1 per COC; P < 0.001). At 6 h IVM, BPS had no significant effect on oestrogen receptors (ESR1, ESR2, GPER) transcripts in CC but 10 nM BPS decreased mRNA expression of P4 receptor (PR) (0.00647 ± 0.00145; P = 0,005) compared with control (0.01165 ± 0.00196). Within fertilized COC, 1 µM BPS decreased cleavage rate (47.6%, n = 152) compared with control (54.6%; P = 0.004). Among cleaved embryos, blastocyst rate decreased to 14.2% and 12.5% with 10 nM and 1 µM BPS respectively (n = 26, P = 0.046; and n = 19, P = 0.017), compared with control (21.8%, n = 44). Bisphenol S at a low dose during ovine IVM reduced COC P4 secretion, PR transcript in CC, and cleavage and blastocyst rates. Our data suggest that BPS at an environmental dose (10 nM) negatively affects early developmental oocyte competence. Studies are ongoing to investigate the effect of BPS on Day 6 embryo cell number and on the ERK1/2 signalling pathway in oocyte.