The Child Behavior Checklist for Ages 2-3 (Adelbach et al., J. Abnorm. Child Psychol. 15;629-650; 1987) was completed by mothers of 229 pairs of twins (mean age = 33 months). Using the two broad-band groupings of Internalizing and Externalizing described by Achenbach et al. (1987), various models to estimate genetic and environmental parameters were fitted using LISREL 7. Model-fitting results showed that the genetic components to the observed phenotypical variation were small and not necessary in the model. Influences from the shared environment, however, could not be dropped from the model without a deterioration in fit. Parameter estimates were not significantly different in boys and girls.