"The reality is that far too many lobbies continue to boycott the register, including many large companies and law firms active in lobbying," said Alter-EU's Olivier Hoedeman.
Companies in the pharmaceutical, nuclear energy, financial sector, agri-business, as well as some Brussels-based think-tanks, are missing from the list.
Altogether, Alter-EU identified 120 companies, many of the world's largest, who actively lobby in Brussels but are not in the register.
Apple Inc, Disney, Time Warner, Rio Tinto, and Monsanto are not in. Some 15 financial institutions, including Deutsche Bank, Goldman Sachs, and UBS have not signed up either.