In two equal periods, using the conventional method of processing bone-marrow cells after Konstantinov and Dobriyanov (1974), we studied the connection between the occurrence of chromosomal aberrations and deviations of phenotype. In the first part of the observation we found 8.5% numerical aberrations in 20 ducklings with retarded motility, torticollis and contractures of the fingers, compared with 1.27% in the control group with normal phenotype. In the second part of the observation the occurrence of aberrations in ducklings with defective phenotype of the same breeding dropped considerably to 1.7% and in the control group to 0.54%. The statistical evaluation of the results showed statistically significant differences (chi (4)2 = 56.78; p less than 0.05, and/or chi (4)2 = 27.74; p less than 0.05).