A high incidence of haploid/diploid chimerism in chick embryos from strains of chickens selected for large size was postulated to be caused by the propensity of such hens to ovulate erratically. To test the hypothesis karyological analysis was made of embryos in eggs containing 1 or greater than 1 yolk. The eggs were from a line selected for multiple ovulation for 20 generations. Double and multiple-yolk eggs are a manifestation of an irregular ovulatory pattern. Ova in multiple yolk eggs were significantly less fertile and significantly fewer embryos survived to 18 h of incubation than single ovulated ova. In the sample of 342 embryos analysed, only 2 forms of heteroploidy occurred in frequencies of greater than 1.2%; 2n/4n mosaicism (5.8%) and 3n (5.0%). Only triploidy occurred significantly more frequently in eggs containing greater than 1 yolk (7.0%) than in single yolk eggs (none). The overwhelming majority of 3n embryos had a digynic origin (i.e from ova with 2 maternal pronuclei), as inferred from the sex chromosome complement. Erratic ovulation therefore resulted in suppression of second polar body extrusion leading to digynic triploidy. Multiple yolks had no effect on dispermy, the primary cause of 1n/2n chimaeric embryos, in single-yolked chicken eggs.