Sustainability transitions often involve policy conflicts, especially when incumbents perceive threats to their business models, and conflicts involve framing of competing positions in the public sphere. Using the case of the growth of distributed solar energy in the U.S. electricity sector, this study shows how the concepts of industry and political opportunity structures can help to explain variation in discursive strategy. First, when incumbents perceive the growth of the challenger as a threat, the industry opportunity structure closes, the volume of framing activity in the public sphere increases as contention grows, and the differentiation in framing (the ratio of pro- to anti-transition frames) between incumbents and challengers increases. Second, with respect to the political opportunity structure, the selection of frame types (environmental versus economic-consumer) varies in relationship to control of the government by conservative or progressive parties. Theoretical implications for transitions studies are discussed.