The chromosome complements of cultured cells of the Falconidae are characterized by their low values: 2n=52 in Falco tinnunculus. 2n=68 in Buteo buteo, 64 in an unknown species (Accipiter nisus ?). The Strigidae have high chromosome numbers: 82 in Strix aluco and Athene noctua, 92 in Tyto alba. In some species the boundary between macro- and microchromosomes is obsolete and in Tyto alba all the chromosome pairs but one are acrocentric. — In all birds examined the female heterogametic sex has two heterochromosomes ZW. In two cases the Z is the fourth or fifth largest as in other birds, in three cases it is the biggest, but the percentual ratio between the length of the Z and that of the haploid set is rather constant. — In the subclass Carinatae. the distribution of the haploid number frequencies is not normal. It is postulated that there are some factors limiting the random scattering of the values.