The chromosomes of 344 early chick embryos (16 to 18 h of incubation) were examined. These embryos were derived from two related lines of broiler-type stock, a line selected for rapid growth rate and a companion line that had been random bred as controls. Of the 344 embryos examined, 282 were obtained from the rapid growth line and 62 from the random-bred controls. A high frequency, 38 of 344, or 11.0%, of these embryos were found to be either entirely or partially composed of cells with abnormal karyotypes. The frequencies of these abnormal embryos in the two lines were 36 of 282(12.7%) in the rapid growth line and 2 of 62 (3.2%) in the control line. Abnormalities observed included: one case of a trisomic/monosomic mosaic involving chromosome No. 1, two haploid embryos, four triploid embryos, four haploid/triploid mosaics, seven haploid/diploid mosaics, sixteen diploid/tetraploid mosaics, one haploid/ diploid/tetraploid mosaic, one haploid/diploid/tetraploid/octaploid mosaic, one diploid/ tetraploid mosaic which also involved sex-chromosome aneuploidy (2A/4A with ZZW/ZZZZWW), and one case of chromosomal rearrangement. Examination of the pedigrees of the abnormal embryos revealed that several mating pairs had produced a number of embryos with similar abnormalities.