A transparent, fireproof thermoplastic composition which is free from halogen compounds and includes a polyamide-block graft copolymer formed by a polyolefin backbone and, on average, at least one polyamide graft. The grafts are attached to the backbone by the radicals of an unsaturated monomer (X) that has a function capable of reacting with a polyamide, and the radicals of the unsaturated monomer (X) are attached to the backbone by grafting or co-polymerisation from the double bond thereof. The composition includes, as a weight percentage of the total composition: 90 to 99 wt % of the polyamide-block graft copolymer, and 1 to 10 wt % of metal salts of phosphoric acid.