Food habit of the Malagasy spiny tailed iguana, Oplurus cuvieri, was investigated based on its fecal samples. This lizard belongs to the endemic Malagasy family Opluridae, for which natural diet had been poorly studied. We focused on the extent of ant-eating and herbivory because these traits are dietary characters often evolved in iguanian lizards. Among 65 fecal samples examined, 73.8% and 40.0% contained ants and plant matter, respectively. The plant matter included small twigs, leaves, flower buds, and fruit seeds. Ants tended to be found more frequently in the dry season than in the rainy season. There was a significant negative correlation between snout-vent length of the lizard and the proportion of ants in its feces. Our results, coupled with the previous direct observations on foraging behavior of the lizard, suggest that O. cuvieri is primarily a sit-and-wait predator largely depending on ants, but also occasionally exploits plant matter by active foraging.