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In the insect rapid cold-hardening response, survival at subzero temperatures is greatly improved by a brief pre-exposure at a milder temperature. It is predicted that insects with minimal cold tolerance capabilities living in variable environments should use rapid cold-hardening to survive sudden cold snaps. This is tested in Afrinus sp., a beetle that lives in an exposed habitat on rock outcrops in the Karoo Desert, South Africa, where microclimate temperatures drop infrequently to below freezing. Afrinus sp. shows a significant rapid cold-hardening response: survival of a 2-h exposure to -6.5 degrees C is much improved after pre-exposure to -2 degrees C, to 0 degrees C with a 2-h return to the rearing temperature, and to 40 degrees C, but not after pre-exposure to 0 degrees C. Little is known about the mechanism of the rapid cold-hardening response, although the data suggest that rapid cold-hardening may be mediated via several different mechanisms.