Theory suggests that secondary sexual traits should honestly reflect males' immunocompetence. We tested whether the activation of a male's immune system has an effect on the female's mate choice in Tenebrio molitor. We found that after a single parasite-like activation of their immune system by a nylon monofilament, males did not suffer from increased mortality but their ability to attract a mate was reduced. However, after a repeated immune challenge, the females more often chose the treatment males, although these males later suffered a high mortality rate, suggesting terminal investment in sexual signalling. The reduced encapsulation response against the nylon implant and reduced survival of these males suggest they did not invest in recovery of their immune system. Our results suggest that under terminal investment males preferred by females may signal their condition dishonestly. (C) 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.