The acoustic signals of insects are examples of sexually selected traits predominantly used by females to identify, locate and choose between potential mates. In addition to the conspicuous calling songs found in many groups, many male Orthopterans also produce a distinct courtship song when in close proximity to a female. It has been suggested that these courtship songs are indicators of male quality and may be used by females to discriminate between potential mates, but little evidence is available. We measured temporal characteristics of courtship song in the cricket Teleogryllus oceanicus and three different aspects of the immune response of the same males. The relations between courtship song structure and immunity appeared to be too weak to be biologically important and hence it seems unlikely that females could use only the song parameters we measured to discriminate in favour of mates with higher immunity. However, when we compared the propensity of males to sing and the structure of their courtship songs before and after they were forced to mount an immune response, we found strong effects of our immune challenge procedure. When females were played songs manipulated to sound like those produced by males with high or low encapsulation abilities (a measure of immunity), they responded more rapidly to the songs corresponding to a high immune response.