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The incidence of spontaneous avian spondylopathy was maintained between 58 and 66% in the progeny of one flock (Flock A) and less than 40% in the progeny of another (Flock B), by proper selection and breeding for 4 generations. Spondylolisthesis, the major type of spondylopathy present, was observed at the first and sixth vertebral levels. Spondylolisthesis was progressive in nature and consisted of a ventrodorsal rotation of the 1st and/or 6th thoracic vertebrae leading to spinal cord injury and paraplegia or partial crippling in some birds. There was occasionally extensive muscle degeneration and connective tissue proliferation around the distorted vertebrae and the spinous ligaments bridging the defect were stretched and appeared heavier than normal. Spondylopathy was also observed in 20-day chick embryos. Spondylolisthesis was observed in 33% of the embryos examined from Flock A-G2 and the other conditions present consisted of scoliosis and abnormal lordosis and kyphosis. Using a grading scale of +1 (mild) to +3 (severe), all cases of embryonic spondylopathy were graded as +1. The back defects present in Flock B-G2 embryos were entirely due to abnormal kyphosis.