A brother and sister of barn owl Ttyo alba guttata originating from a population in the Czech Republic bred in an aviary from 1994 to 1998. In eight breeding attempts, the inbred pair produced 30 nestlings out of which 7 (23%) were extremely dark on the underparts. Since this phenotype is very rare in barn owl populations and the proportion of dark nestlings was very close to phenotypic ratio 1:3, we consider that a recessive allele caused the expression of an extreme dark coloration and that one grand-parent of the dark barn owls possessed one copy of this recessive allele.