The flameproofed, rigid, isocyanate-based foams, in particular rigid polyurethane and polyisocyanurate foams, are produced by reactinga) organic and/or modified organic polyisocyanates withb) at least one relatively high-molecular-weight compound containing at least two reactive hydrogen atoms, and, if desired,c) low-molecular-weight chain extenders and/or crosslinking agents, in the presence ofd) blowing agents,e) catalysts,f) flameproofing agents, and, if desired,g) further auxiliaries and/or additives,wherein the flameproofing agent is a combination of at least one liquid flameproofing agent which is reactive toward isocyanates and at least one solid flameproofing agent.