Several anurans have morphological, physiological, and behavioural antipredator mechanisms (Toledo and Haddad, 2009; Toledo et al., 2011; Mailho-Fontana et al., 2013; Ferreira et al., 2019). Polyphenism is the change of skin colour patterns in response to an environmental stimulus (e.g., antipredator response; Hanlon et al., 1999). Some anurans have chromatophore cells to promote instantaneously or gradually colour changing (Toledo and Haddad, 2009; Ferreira et al., 2019). This strategy is usually associated with camouflage to resemble the environment background favouring the use of different substrates (Toledo and Haddad, 2009), which may decrease the risk of predation (Ferreira et al., 2019). Also, polyphenism of some arthropods is associated with facultative aposematism, were the species can assume aposematic colours under specific environmental conditions (Sword, 2002; Despland and Simpson, 2005). Here we follow the definition of Toledo and Haddad (2009) and Ferreira et al. (2019). Polyphenetic behaviour has been recorded in several species within the Phyllomedusidae. Pithecopus azureus (Cope, 1862), Pithecopus megacephalus (Miranda-Ribeiro, 1926), Pithecopus rohdei (Mertens, 1926), Phasmahyla cochranae (Bokermann, 1966), Phasmahyla guttata (Lutz, 1924) and also Phasmahyla jandaia (Bokermann and Sazima, 1978) change skin colour from purplish during night activity to greenish during daytime resting (Toledo and Haddad, 2009). Phasmahyla guttata was the first species of the family observed displaying dynamic polyphenism (i.e., fast colour change) (Machado et al., 2015). The genus Phasmahyla Cruz, 1991, is composed by eight species endemic to the Atlantic Forest (Frost, 2020), breeding in mountain streams from eastern Minas Gerais, southern Bahia to eastern Paraná State (Cruz et al., 2008; Pereira et al., 2018; Frost, 2020). Phasmahyla exilis (Cruz, 1980) was described from Santa Teresa, State of Espírito Santo. Its known distribution is limited to a few localities in the State of Espírito Santo and southern Bahia (Frost, 2020). At 9:00 pm, 25 January 2019, during a nocturnal herpetological survey in the Estação Biológica de Santa Lúcia, Municipality of Santa Teresa, State of Espírito Santo, southeastern Brazil (19.9749ºS, 40.5317°W; WGS84, 720 m a.s.l.), six juvenile individuals of Phasmahyla exilis were found perched on herbaceous vegetation at a stream bank. At first, the individuals were light green with black spots (Fig. 1A). Approximately two minutes after handling, all six individuals started changing colouration by displaying red/orange spots on Herpetology Notes, volume 13: 813-816 (2020) (published online on 05 October 2020) Aposematic polyphenism: Antipredator mechanism of juveniles of Phasmahyla exilis (Cruz, 1980) (Amphibia, Anura, Phyllomedusidae), in southeastern Brazil Lucas Rosado Mendonça 1 , Ubiratã Ferreira Souza 2 , Maria Eduarda Bernardino Cunha 3 , Leticia Keiko Watanabe 4 , Thiago Silva-Soares 5 , and Alexander Tamanini Mônico 6,* 1 Departamento de Biologia Animal, Laboratório de Herpetologia, Museu de Zoologia João Moojen da Universidade Federal de Viçosa, Vila Giannetti 32, Viçosa, Minas Gerais 36570-900, Brazil. 2 Departamento de Biologia, Laboratório de Estudos Herpetológicos e Paleoherpetológicos da Universidade Federal Rural de Pernambuco, Rua Dom Manoel de Medeiros s/n, Recife, Pernambuco 52171-900 Brazil. 3 Departamento de Zoologia, Instituto de Biociências, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves 9500, Porto Alegre, Rio Grande do Sul 91509- 900, Brazil. 4 Departamento de Ciências Biológicas, Universidade Federal do Espírito Santo, Campus Goiabeiras, Avenida Fernando Ferrari 514, Vitória, Espírito Santo 29075-910, Brazil. 5 Projeto Herpeto Capixaba, Rua Silvino Grecco 813, Vitória, Espírito Santo 29090-230, Brazil. 6 Instituto Nacional de Pesquisas da Amazônia, campus V8, Avenida André Araújo 2936, Manaus, Amazonas 69067-375, Brazil. * Corresponding author. E-mail: alexandermonico@hotmail.com