DNA interstrand crosslinks (ICLs), highly toxic lesions that covalently link the Watson and Crick strands of the double helix, are repaired by a complex, replication-coupled pathway in higher eukaryotes. The earliest DNA processing event in ICL repair is the incision of parental DNA on either side of the ICL ("unhooking"), which allows lesion bypass. Incisions depend critically on the Fanconi anemia pathway, whose activation involves ubiquitylation of the FANCD2 protein. Using Xenopus egg extracts, which support replication-coupled ICL repair, we show that the 3' flap endonuclease XPF-ERCC1 cooperates with SLX4/FANCP to carry out the unhooking incisions. Efficient recruitment of XPF-ERCC1 and SLX4 to the ICL depends on FANCD2 and its ubiquitylation. These data help define the molecular mechanism by which the Fanconi anemia pathway promotes a key event in replication-coupled ICL repair.