The hydrodynamic drag and thermophoretic properties of fractal-like aggregates of spheres in the free-molecular flow regime are examined using a Monte Carlo method. A modification of the sampling procedure that is used to assign velocities to the computational molecules is developed, and allows for a relatively fast and accurate determination of the particle transport properties for small-Mach number (i.e., slow-moving) conditions. Aggregate targets in the simulation are generated using an algorithm which mimics cluster-cluster aggregation, and simulations are performed on aggregates with a range of fractal dimensions and structure factors. The results indicate that for fractal dimensions in the range of D-f = 1.7-2.0, the hydrodynamic radius of the aggregate scales as a(m) similar to aN(S)(0.47), in which a is the sphere radius and N-S is the number of spheres. Results also show that the thermophoretic velocity of a randomly-oriented aggregate will be slightly larger than that of the isolated spheres. The enhancement in velocity increases with N-S, yet for relatively large-scale aggregates with N-S = 3000 the increase is only around a factor of 1.08. (C) 2004 Elsevier Ltd. All rights reserved.