Inmates' informal code often ascribes low status to persons perceived as passive homosexuals. We use longitudinal data to investigate whether prison experience contributes to anti-gay beliefs. We find that prison experience prompts a higher level of anti-gay sentiments among males and their families, while no discernible difference exists before incarceration. We find no effect for female ex-prisoners. We confirm that the results are not driven by pre-incarceration trends, changes in trust and social capital, socioeconomic status, mental health, masculinity norms, and other potential alternative explanations. Our study sheds light on the overlooked role of prisons as a significant contributor to the propagation of anti-gay attitudes.