Seventy-seven males of Hypsiboas prasinus from 2 Atlantic forest fragments in the municipalities of Botucatu and Jundiaí, São Paulo State, Brazil, were examined for endoparasites. The frogs were captured in summer (January until March) and winter (July/August) of 2008 and 2009. Thirty-three males (75%) from Botucatu were infected with Rhabdias cf. fuelleborni, cosmocerciid nematodes, and Cylindrotaenia americana. Twenty-five tree frogs (78.5%) from Jundiaí were infected by Rhabdias cf. fuelleborni, Physaloptera sp., and cosmocerciid nematodes. Only cosmocerciid nematodes presented a statistically significance difference in prevalence (z  =  4.345; P < 0.001) and mean abundance (t  =  562.0; P < 0.001) between Botucatu and Jundiaí during the winter. Also, the cosmocerciids exhibited higher mean abundance (t  =  196.0; P  =  0.034) in winter when compared with summer at the Jundiaí site. Moreover, to our knowledge, this is the first report of C. americana in the Brazilian Hylidae. This study presents 4 new records of nematodes in H. prasinus.