Secondary sexual traits have been suggested to provide reliable signals of a male's ability to resist infection by agents of disease. The immunocompetence handicap hypothesis provides a potential mechanism for reliable signalling in the form of a trade-off between expenditure on trait expression and expenditure on immunity. Thus, males resistant to disease can spend more resources on their sexual signals. Examination of calling song parameters in a natural population of the cricket Teleogryllus commodus revealed that males scoring higher on the third principal component for song had significantly lower ability to encapsulate a foreign object. This component of immune function was associated with syllables of longer duration in both the trill and chirp elements of the song. Males with longer syllables in their song had a lower encapsulation ability. Syllable duration is known to influence phonotaxis by female T. commodus. Although the effect was only weak, our data suggest that females may base their choice of mate on reliable information contained within the temporal properties of male calls. Our study thus demonstrates a connection between sexual signalling and immune function in a natural population of insects and lends support to the. immunocompetence handicap hypothesis. (c) 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.