1. Twenty-eight pairs of monozygotic and eighteen pairs of dizygotic twins were tested for their ability to taste phenylthiocarbamide (P.T.C.) by the method of Harris & Kalmus (1949). 2. Much greater variance in threshold levels was detected within the DZ than within the MZ pairs and this confirms the genetic origin of most variation in ability to taste this substance. 3. Variance in threshold levels within MZ pairs is of the same magnitude as the variance of reported duplicate measurements on the same individuals. 4. DZ twin pairs were classified as either concordant or discordant in their tasting ability. Variance within concordant DZ pairs is significantly greater than within the MZ pairs and it is shown that this difference can be accounted for in terms of the incomplete dominance of the T allele reported by Kalmus (1958). However, the data do not exclude the possiblity that this greater variation in threshold levels is partly due to multiple alleles or background genetic variation. 5. The frequency of the t allele in Australians of European descent in estimate at 0-52 plus or minus 0-06.