We have previously demonstrated that flour beetles, Tribolium confusum, are more attracted to the feces from rats with patent Hymenolepis diminuta infections than to feces from uninfected rats. The objective of this study was to determine if this effect is due to a volatile attractant. Volatile substances emitted by feces from H. diminuta-infected rats or from uninfected controls were collected by aspirating fresh rat feces, while trapping the volatiles on Porapak Q (a solid adsorbant). The volatiles were eluted from the Porapak Q with diethyl ether, and the relative attractancy of the volatiles to prestarved beetles was assessed by bioassay. More beetles were attracted to volatiles of feces from infected rats than to volatiles of feces from uninfected controls (P< or =0.0001). The magnitude of the response varied with the time in the bioassay test arena and also the concentration of the volatiles (P< or =0.0232). When the volatiles we re concentrated by aspirating more boli over a longer period of time, the beetles responded more quickly and in greater numbers to the volatiles of feces from infected rats. The experiments presented here provide the first indication that a tapeworm (H. diminuta) can enhance its chances of transmission by directing the foraging of its intermediate host (T. confusum) through the use of attractive, volatile material released from the feces of its definitive host.