This paper shows how lightpath-based networks can allow challenging,fine-grained parallel supercomputing applications to be run on a grid,using parallel retrograde analysis on DAS-3 as a case study. Detailed performance analysis shows that several problems arise that are not present on tightly-coupled systems like clusters. In particular, flow control, asynchronous communication,and host-level communication overheads become new obstacles. By optimizing these aspects, however, a 10G grid can obtainhigh performance for this type of communication-intensive application. The class of large-scale distributed applications suitable for running on a grid is therefore larger than previously thought realistic.