The Yucatan Wren (Campylorhynchus yucatanicus) is a highly restricted endemic species inhabiting the coastal scrub at the northern portion of the Yucatan Peninsula. We describe the breeding phenology and nesting success of this endangered species from April to September 2007 for a population at Ria Celestun Biosphere Reserve. We found 232 nests of which only 110 (47%) were active at either incubation or nestling stages. Yucatan wrens initiated nest building in late April, but clutch initiation occurred in early June and lasted until the end of July, resulting in a breeding season of 2 months. Nests were exclusively in coastal scrub and the transition between coastal scrub and black mangrove (Avicennia germinans) forest. Eleven species of trees were used as nesting substrate, but three included 75% of all nests found. Clutch size (x¯ ± SD) was 3 ± 1.5 eggs with incubation and nestling periods averaging 16 ± 1.0 days, and 16.5 ± 1.9 days, respectively. Mayfield estimates of daily survival rate for incubation and nestling periods were 0.968 ± 0.005 and 0.975 ± 0.005, respectively with nesting success of 46%. The average number of fledglings per successful nest was 2.5 ± 1.3. Predation was the main cause of nest mortality accounting for 54% of the active nests. Parental care was provided by both parents, but participation of a third individual feeding nestlings was recorded at three nests, providing evidence for occasional cooperative breeding. Increasing human development in the coastal region of the Yucatan Peninsula may represent a serious threat to conservation of the Yucatan Wren due to habitat restriction and high dependency on three species of trees as nesting substrate.