Unsteady 3-D computational fluid dynamics (CFD) results can be very large. Some recent solutions exceed 100 gigabytes. Visualization techniques that access the entire data set will, therefore, be excruciatingly slow. We show that particle tracing in vector fields calculated from disk resident solutions can be accomplished in O(number-of-particles) time, i.e., visualization time is independent of solution size. This is accomplished using memory mapped files to avoid unnecessary disk IO, and lazy evaluation of calculated vector fields to avoid unnecessary CPU operations. A C++ class hierarchy implements lazy evaluation such that visualization algorithms are unaware that the vector field is not stored in memory. A numerical experiment conducted on two solutions differing in size by a factor of 100 showed that particle tracing times varied by only 10-30 percent. Other visualization techniques that do not access the entire solution should also benefit from memory mapping and lazy evaluation.