Butterfly color patterns are built as finely tiles mosaics o f colored scales, each the product o f a single epidermal cell. The overall pattern is composed o f a small number o f pig=ments. Each scale appears to make an all-or-none “choice” to synthesize only one o f this small set o f alternative pigments. Gradual transitions from one color to another on the wing are accomplished by graded changes in the proportions o f discretely colored scales, and areas o f intermediate coloration are made up o f a random salt-and-pepper mosaic o f discretely colored scales. In transition regions, and at boundaries between colors, scales also exhibit sto= chastic patterns o f pigment expression. A model for a stochastic mechanism for gene expres= sion is presented that can account for the observed stochastic patterns o f pigmentation. This model shows that stochastic gene expression provides a simple mechanism for establishing a step-like, or threshold, response to a spatially graded signal. Conditions that affect the steep= ness o f the threshold are examined. It is shown that stochastic gene expression also results in the emergence o f interesting patterns o f dominance among the alleles o f a gene.