<jats:p> A small Neotropical frog, <jats:italic>Dendrobates pumilio</jats:italic> , undergoes interpopulational variation in color, degree of toxicity, size, and habits. Differences in body coloration encompass the visible spectrum from red to blue, as well as achromatic black and white. There are wide variations in the degree of toxicity, but these variations are not correlated with supposed warning colors. Extracts of skin yield two toxic compounds characterized as steroidal alkaloids with molecular formulae C <jats:sub>19</jats:sub> H <jats:sub>33</jats:sub> NO <jats:sub>2</jats:sub> and C <jats:sub>19</jats:sub> H <jats:sub>33</jats:sub> NO <jats:sub>3</jats:sub> . The rapid rate of divergent evolution among populations of this frog may result from isolation and chance restriction of original heterozygosity, with subsequent selection acting on different and greatly limited mixtures of alleles.