Agalychnis annae is a species endemic to Costa Rica and Panama with populations shrinking or disappearing from several locations. We studied the reproduction of a relict population of A. annae in a 1,300 m2 natural pond in Costa Rica during 2007–2008. We marked adults with a visible implant alphanumeric tag injected in the thigh. We measured reproductive effort considering calling males per night and per hour, number of pairs in amplexus, and number of egg masses. We summed 366 individual observations: 57% juveniles, 39% adult males and 4% adult females. Average SVL was 29 mm (juveniles), 63 mm (males) and 78 mm (females). Sex ratio (males:females) was 4.85:1, while operational sex ratio varied between 1–6 males per female when both sexes were present at the pond. Maximum estimated census population size varied between 27 and 59 individuals, with a Ne = 43.11. Calling activity occurred mostly from 1800 h – 2000 h, and we counted 65 egg masses. Number of adults and calling males were positively correlated with accumulated rainfall and average temperature. Number of egg masses was positively correlated with the number of adults, accumulated rainfall, and average temperature. The causes of the long-term population decline must be identified to conserve A. annae and similar species, as well as establishing long-term monitoring programs. Required conservation actions include: translocation individuals from risky areas, creation of artificial ponds to increase ecological connectivity, elimination of introduced species, particularly in small habitat remnants, and establishment of ex situ conservation programs.