Seasonal changes in the frequencies of green and brown Pacific Treefrogs (Pseudacris regilla) were recorded at a site in southern California. Changes in the proportions of green and brown individuals in the population appear to track seasonal changes in the availability of green and brown background substrates. We show that the change in the frequencies of green and brown frogs is not just caused by a change in the proportion of frogs with fixed green and brown body colors but also by the ability of a newly described morph to change color. The findings add further support to the hypothesis that maintenance of this complex color polymorphism likely involves a response to seasonal change in the abundance of green foliage, perhaps driven by differential predation on green and brown frogs that are either plastic or fixed in color.