Cytogenetic analysis was carried out in a total of 59 females and 45 males from 48 litters of Merino Booroola sheep. Karyotypes were investigated only in sheep coming from multiple heterosexual pregnancies. The study was performed over three consecutive years. Lymphocyte 54,XX/54,XY chimerism was identified in 16 ewes and 16 rams, i.e. 30.8% of the karyotyped group of animals and 10.32% of all animals born alive in the flock.