Type locality: San Isidro, Buenos Aires, Argentina.
Larvae and pupae of Culex brethesi have been collected in large swamp marsh depressions with grasses, sunlight, pH between 5.50 and 6.79, together with larvae of Aedeomyia squamipennis, Nyssorhynchus albitarsis, Anopheles annulipalpis, Culex idottus and species of Mansonia, Psorophora, and Uranotaenia. Larvae were also collected in ponds and pools with or without grass. Some specimens were collected in association with larvae of Culex dolosus and Culex maxi.