Trypanosoma cruzi was transmitted directly between triatomines by cannibalism or coprophagy. Different conditions involving cannibalism that excluded coprophagy were studied in Dipetalogaster maximus. Infections occurred if an uninfected donor bug sucked infectious blood and if this blood was taken up from the stomach by a cannibalistic bug. If the donor was infected and sucked uninfected blood afterwards, the source of the uninfected blood determined the transmission rate: If the uninfected blood originated from mice, many cannibalistic bugs became infected because complement factors from mouse blood did not lyse T. cruzi in the stomach of the bug. If the uninfected bloodmeal originated from chickens, cannibalistic bugs occasionally became infected, even though chicken blood is known to lyse all stages of T. cruzi in the stomach. Experiments on coprophagy provided the first conclusive demonstration that transmission of T. cruzi occurs between individual Triatoma infestans, as a result of coprophagic behaviour alone, and excluding the possibility of cannibalistic transmission.