Variegated patterns on the caudal fin are a common and popular trait in guppy strains commercially cultured in Singapore. Gene control of this highly variable mosaic pattern of black spots and patches of different shapes and sizes on a brightly colored tail fin was elucidated by reciprocal crosses between the Green Variegated (GV) strain and wild-type (WT) stock. F₁ progenies were produced by single-pair crossing between GV and WT, while the F₂ generation was obtained from full-sib mating between F₁ males and F₁ females. Data for the F₁ and F₂ generations were segregated according to phenotypes and sex, and tested by chi-square analyses. Inheritance of variegated tail patterns appears to be determined by a single locus on the X- and Y-chromosomes. Genotypes of males and females of the GV strain are proposed to be X _VarY_Var and X_VarX_Var, respectively. The allele for variegated tail patterning, Var, is dominant over that of the wild-type, Var , which does not exhibit these patterns. Recombination frequency between the Var locus and sex-determining region (SdR) in male guppies was estimated to be about 1.9% (map distance ≈1.9 map units). The segregation and mode of inheritance of the Var gene are illustrated by genetic models.