Several studies have noted that the International Seabed Authority (ISA) scores low on public participation. However, none have studied the eforts of non-governmental organizations to exert infuence on the ISA’s rulemaking processes. I examine how environmental NGOs and private mining contractors attempt to sway one narrow, but existential, part of the ISA’s draft exploitation regulations between 2014 and 2019: the defnition of “serious harm” to the marine environment. Although environmental NGOs appear to have been more successful in infuencing that defnition, the interests of private contractors may still prevail. Despite the eforts of environmental NGOs, the term “serious harm” remains largely undefned, allowing for more subjectivity and fexibility in interpretation. This challenge is exacerbated when combined with current institutional weaknesses and limited scientifc expertise within the ISA. Ongoing negotiations and recent developments may, however, alter this outcome.