Twin ovules, not previously reported for Prunus, occurred in from 5 to 12% of Windsor sweet cherry pistils. Rarely, twin seeds have been observed in mature fruits of other genera. The conclusions of earlier workers that twin seeds arise through fusion or fission of ovules or nucelli are not supported by the present study. Regardless of whether neither, one, or each ovule was a twin, two ovules were present in every fruit examined. Observations on the ovule development in Prunus avium indicate that twin ovules arise by the differentiation of separate nucelli in a single ovule primordium.