Summary. Holstein–Friesian cattle heterozygous for the deficiency of uridine monophosphate (UMP) synthase have half-normal activity of UMP synthase. The homozygous recessive genotype would result in little or no activity, has not been observed among live animals and apparently leads to embryonic mortality at ∼Day 40 of gestation. Activity of UMP synthase averaged 2·74 ± 0·61 units/mg protein for 19 obligatory normal embryos (from normal × normal matings). Activity for 18 embryos from heterozygote × heterozygote matings yielded three non-overlapping groups as follows: (i) five presumed normals with > two-thirds normal activity, (ii) ten apparent heterozygotes with one-third to two-thirds normal activity and (iii) three putative homozygous recessive embryos with < one-third normal activity. The distribution among these groups was consistent with the 1:2:1 ratio expected for autosomal inheritance. Conception of embryos homozygous recessive for this disorder was demonstrated.