This study investigates the impact of environmental costs on equity and assets of quoted manufacturing firms in Nigeria, employing ex-post facto and regression research designs using secondary data from 2011-2018. It concludes that environmental costs significantly affect both return on equity and return on assets, recommending firms invest in environmental issues for better stakeholder relationships and performance. The study underscores the importance of environmental disclosure in shaping corporate accountability and sustainability amidst increasing societal and regulatory pressures.