The expression of carcinin, a crustin-type antimicrobial protein, in the crab, Carcinus maenas, was studied following in vivo challenge with Planococcus citreus, a Gram-positive bacterium known to be killed by the encoded protein. Real-time PCR analyses reveal that injection of P. citreus failed to elicit any significant changes in expression at 0-24h post-injection although there was a small, but significant, down-regulation at 84h in crabs held at 15 degrees C but not those at 5 or 20 degrees C. By contrast, un-injected crabs held at various temperatures between 5 and 20 degrees C, showed significantly up-regulated expression at 5 and at 20 but not 10 degrees C compared with controls at 15 degrees C. Thus expression of carcinin seems to be affected by temperature, especially when the animal is close to the edges of its physiologically tolerated thermal range.