Phyto-oestrogens such as isoflavones are natural compounds that can profoundly affect reproductive function. We tested whether including isoflavone compounds (genistein, biochanin A, formononetin) in the maturation medium would affect the outcomes for ovine oocytes in vitro. A factorial design (3 isoflavones x 5 concentrations: 0, 2.5, 5, 10, 25 µg mL-1) was used and the entire protocol was repeated four times. Cumulus-oocyte-complexes were randomly allocated to the treatments, then fertilized and cultured in vitro. Compared with control (0 µg mL-1), the lower concentrations of isoflavone (2.5, 5 and 10 µg mL-1) had no detectable effect on the rates of cleavage or embryo development, or on embryo total cell counts (TCC). However, the highest concentration (25 µg mL-1) of all three isoflavones exerted a variety of effects (p < 0.05): genistein decreased cleavage rate, blastocyst rate and blastocyst efficiency (blastocysts produced per 100 oocytes); biochanin A decreased cleavage rate and blastocyst efficiency; and formononetin decreased blastocyst rate and blastocyst efficiency. Biochanin A (25 µg mL-1) reduced embryo TCC specifically at the hatched blastocyst stage (P < 0.05). We conclude that the presence of isoflavones at 25 µg mL-1 during in vitro maturation decreases the cleavage rate and inhibits blastocyst hatching.