Rice leaves accumulate serotonin and its hydroxycinnamic acid amides in response to pathogenic attack. Serotonin is incorporated into cell walls by the oxidative polymerization by peroxidase reactions, and forms brown deposition that reinforces the cell walls. Generally, the phenolics involved in the reinforcement of cell walls originate form phenylalanine. Thus, the incorporation of serotonin, which is derived from tryptophan, into cell walls represents a new mechanism for the reinforcement of cell walls. Here, we examined the induction of serotonin accumulation in some grass family plants to investigate the generality of this response. The conidia of Bipolaris oryzae and B. maydis were inoculated to the leaves of rice, barley, wheat, oat, maize, sorghum, proso millet, Japanese barnyard millet and foxtail millet. After a 48-h incubation, the contents of serotonin and its hydroxycinnamic acid amides were determined by LC-MS/MS analysis. The analyzed species were classified into two clades in the Poaceae, BEP clade (rice, barlay, wheat and oat) and PACCMAD clade (maize, sorghum, proso millet and Japanese barnyard millet and foxtail millet). The accumulation of serotonin was detected in all examined species although its amount greatly varied depending on the species. Hydroxycinnamic acid amides also accumulated in the species in both clades. These findings suggest that the accumulation of serotonin and its amides was a trait of the common ancestor of BEP and PACCMAD clades.