Twin and pedigree data are presented which indicate that relative length of hallux and adjacent second toe is based on simple Mendelian genetic factors. The normal anatomical trait involves two alleles and a single autosomal locus. The factor for relatively long hallux is recessive to the one for relatively long second toe. There is no evidence of sex influence. Penetrance is 100%. Expressivity is variable, and bilateral discordance is observed in a minority of heterozygous subjects. In every such case, the dominant genotype is expressed in the right foot. The recessive and dominant phenotypes occur, respectively, in 76% and 24% of the greater Cleveland area Caucasian population. Gene frequencies in this population are 87% and 13%, respectively, for the recessive and dominant allele.