1. This study was undertaken with two points in view: First, to determine the number of chromosomes and second, to discover the mechanism for sex-determination and sex-linkage in the duck as a representative of the aves.2. Material from thirteen individuals was used, seven males and six females.3. Chromosome counts were made on entire cells, both somatic and spermatocytic.4. In the somatic cells there appear to be seventy-six chromosomes for the male and seventy-seven chromosomes for the female. There is present in the cells of the female a long unpaired chromosome which is not found in the cells of the male. There is reason to suppose that there are probably among the remaining six largest chromosomes two more unpaired chromosomes, one of which, the largest, is probably homologous to the largest pair (sex-linkage) of chromosomes in the male complex, while the other, it is thought may be some one of the five remaining long chromosomes.5. There appear to be 38 bivalents in the primary spermatocytes of the male. These agree with the pairs of somatic chromosomes in size gradations. Most of the chromosomes in the spermatocytes appear to be bivalent or tetrad in form.6. Sutures accompanied by constrictions seem constant in position in at least two points in the largest rod-shaped chromosomes. These sutures seem to lend additional pliancy to these regions in the chromosomes.7. Gonomeric grouping of chromosomes occurs in the amnion cells of the duck. It is also thought to be present in the chromosome plates of the gonial cells in the embryo of the chick and the lizard.8. Filamentous linkage between certain of the smaller chromosomes appears to be present in certain stages of the prophase.9. There is reason to believe that the sex-mechanism is of the WwZ-ZZ type similar to that found in the moth Phragmatobia.10. This would give the female more chromatin than the male and yet preserve female digamety, which would bring this type into harmony with the usual conditions found in the XX-Xy type.