A population of the giant leather fern Acrostichum danaeifolium was observed during an 18-mo period at La Mancha (19°36′00″N, 96°22′40″W), Veracruz, Mexico. The study site was 230 m from a brackish-water lagoon, in the understorey of the mangroves, which are dominated by Avicennia germinans (Avicenniaceae). Acrostichum danaeifolium showed a clumped distribution pattern, and one third of the population became fertile during this time. Plants had a mean number of 9.4 ± 0.45 sterile leaves, which developed continuously at a rate of 14.6 ± 0.44 leaves y-1. The mean life span of sterile leaves was 7.7 mo. Although soil water was always available, phenological patterns of fertility, leaf production and leaf growth were strongly correlated with the seasonal climate. Individual leaf growth and leaf size increased significantly during the rainy season. Fertile leaves emerged exclusively from April to August and had a mean life span of 4.1 mo. Spore release was restricted to the wet season, when conditions for germination were favourable. Environmental triggers of fertility are discussed and phenological patterns are compared with ferns and angiosperms from other habitats.