Sexual armaments are usually structures or other traits used during agonistic displays that evolve by intrasexual competition. The role of parasites in their evolution remains obscure. We investigated the relation of infection by three parasites with the agonistic behavior of males Sceloporus occidentalis in male contests staged in the lab. The rivals' both behavior and blue patch chroma best predicted the aggression intensity of focal males. Infections by haemococcidians and ticks also contributed to explain the intensity of aggression in focal males. Lizards with fewer ticks engaged in more intense fights, while lizards with more ticks or haemococcidians were less aggressive. Interestingly, males with lower blue chroma were infected by intestinal coccidians and received more aggression. This study reveals direct roles for ticks and haemococcidians in male competition, and an indirect role of intestinal coccidians mediating sexual selection of a color armament in lizards.