1. The genetical and environmental structure of covariation between finger ridge counts in twin and sibling data has been analysed using the method of Martin and Eaves (1977) adapted from Jöreskog. 2. The model for environmental covariance contains a single factor loading on all ten digits but most environmental variance is specific to each finger. 3. For additive genetic variance there is one common factor loading on all digits. There are also five other independent factors, one for each digit. The thumb factor loads only on the two thumbs but the four fingers load on the finger in question and on the adjacent fingers. 4. A single common factor for non-additive genetic variance produces a considerable improvement in the model. 5. The pattern of genetic effects differs between left and right hands. 6. Although the same model is appropriate for males and females, different parameters estimates are required. 7. The fit of models is sensitive to the scale of measurement.