In this paper, I examine the speculative dimensions of biodiesel production in Northeast Brazil under the National Program for the Production and Use of Biodiesel (PNPB), by focusing on the case of castor oil plant (Ricinus communis) in semi-arid Bahia, a promising flex crop that was said to generate social, environmental and economic benefits, especially for the historically marginalised Northeast peasantry. I argue that speculative practices over castor have relied on state policies and discourses as well as biophysical and biotechnological (im)materialities that are transforming agrarian relations in ways that compromise peasants' control over their (re)productive systems while promoting the expansion of the agroindustrial frontier. By examining the everyday, performative and place-based character of speculative biodiesel practices and their uneven and contested socioenvironmental implications, this paper contributes to current political ecology debates on the nexus between agroenergy development, nature's capitalist valuation and rural dispossession. Resumo: Neste artigo, examino as dimensões especulativas da produc ßão de biodiesel no Nordeste do Brasil, noâmbito do Programa Nacional de Produc ßão e Uso do Biodiesel (PNPB), concentrando-me no caso da mamona (Ricinus communis) no semi arido baiano, uma cultura dita flex ıvel que prometia gerar benef ıcios sociais, ambientais e econômicos, especialmente para os camponeses do Nordeste historicamente marginalizados. Argumento que pr aticas especulativas sobre a mamona se constitu ıram atrav es de pol ıticas e discursos do Estado, bem como de (im)materialidades biof ısicas e biotecnol ogicas que transformaram as relac ßões agr arias, comprometendo o controle dos camponeses sobre seus sistemas (re)produtivos e promovendo a expansão da fronteira agroindustrial. Ao examinar o car ater cotidiano, performativo e situado das pr aticas especulativas de biodiesel, bem como suas implicac ßões socioambientais desiguais e contestadas, este artigo contribui para os debates atuais em ecologia pol ıtica sobre o nexo entre desenvolvimento agroenerg etico, valorizac ßão capitalista da natureza e despossessão rural.