Serotonin (5-HT) is an essential neurotransmitter for many physiological and behavioral processes. Clinically and experimentally 5-HT metabolism can be altered using a class of drugs known as selective serotonin reuptake inhibitors (SSRIs). SSRIs such as fluoxetine have proven effective tools for elevating 5-HT activity and are routinely utilized to study the role of 5-HT in aggressive behavior. The Puerto Rican coquí frog, Eleutherodactylus coqui, is a terrestrial amphibian that exhibits territoriality and paternal care. E. coqui have three different male behavioral modes, territorial (calling), non-calling, and paternal. Territorial male E. coqui aggressively maintain and defend territories by emitting advertisement calls. The objective of this study was to determine if the SSRI, fluoxetine, influences the establishment of the dominant (territorial)-subordinate (non-calling) relationship in male E. coqui. Wild captured adult territorial male E. coqui were grouped into 16 pairs in semi-natural terraria. Pharmacological treatment consisted of injecting (IP) experimental frogs with 25 μl (10mg/kg fluoxetine) of a fluoxetine/saline solution while control males received IP injections of saline. Injections were administered every other day for 20 days while observations continued until 40 days. Results indicated significantly higher numbers of advertisement calls emitted from control males after 20, 30, and 40 days. Additional, 13 of 16 control males emitted significantly more advertisement calls and became the territorial male. Fluoxetine treatment significantly reduced advertisement calling and territorial behavior in E. coqui males. These findings demonstrate that in E. coqui the 5-HT system has profound influence on male territorial and social behavior. Copyright © 2014 Elsevier Inc. All rights reserved.