There is an optimal size for the delivery of small asteroids from Mars to the Earth by Yarkovsky thermal drag. Basaltic asteroids with radii of about 6 m take on the average 185 million years (Myr) for their semimajor axes to shrink by 0.52 AU, assuming circular orbits and ignoring planetary perturbations and collisions. All other sizes take longer. Bigger objects are slower because they are more massive, and smaller objects are slower because they are more isothermal. These results are based on treating the asteroids as spheres and solving the heat conduction equation using spherical Bessel functions. The small near-Earth asteroids show a concentration of sizes in the thermal drag range; thus some of them may come from Mars as survivors of gravitational mechanisms which eliminate them on the 10 Myr timescale. The possible role of thermal drag in Mars-Earth delivery will remain speculative until it is included in numerical integrations of the orbits of small asteroids. ¿ 1998 American Geophysical Union