In turtle species with temperature-dependent sex determination, hatchling sex ratios skewed toward males could threaten populations where females are vulnerable to harvest. We tested the efficiency of black plastic covers in producing more female hatchlings from nests of the six-tubercle Amazon River turtle (Podocnemis sextuberculata) at the Mamirauá Reserve, Amazonas, Brazil, during September to November 2003. Covered nests produced 37% more females and midnest average temperatures were 1° C higher than control nests. Incubation period was 3 days shorter, but survivorship was 12% lower in nests with black plastic. This technique could be an inexpensive, local alternative for short-term sex-ratio manipulation in endangered male-biased populations with temperature-dependent sex determination.