This study aimed to clarify genetic and environmental contributions to Carabelli trait variation on permanent first molar teeth in a large sample of South Australian twins. Estimates of polychoric correlations were obtained between pairs of monozygous (MZ) and dizygous (DZ) twins for Carabelli data and various gene-environment models fitted by a weighted least-squares approach. The favored model included additive genetic effects together with both a general environmental component and an environmental effect specific to each side. An estimate of heritability around 90% indicated a very strong genetic contribution to observed variation. The pattern of correlations for MZ and DZ data suggested that further studies involving other types of relatives would be worthwhile for detection of possible non-additive genetic effects of dominance or epistasis.