Abstract The effects of salinity on the structure of salt glands and salt secretion were determined by comparing adult Avicennia marina trees growing in the field at two contrasting salinities: seawater and hypersalinity. Electron microscopy was used to determine gland frequency and ultrastructure. Ecophysiological measurements included ion analyses of soils and leaves, soil and xylem water potentials and photosynthesis. In the hypersalinity site, salt gland frequency was 47% lower than that at seawater conditions. Salt glands in the hypersalinity site were sunken within crypts, partially collapsed, degenerated, and covered with salt crystals. In the hypersalinity site, salt secretion during the day was lower than that in the seawater site by 33% ( p < 0.05), while there were no differences in night secretion. In both sites, salt secretion was higher at night and lower during the day. In the hypersalinity site, the cytoplasm of the salt glands had fewer ribosomes and mitochondria and larger vacuolar and vesicular volume than at the seawater site. CO 2 exchange, conductance, transpiration and intrinsic Photosystem II efficiency. were significantly lower in the hypersalinity site than in the seawater site. Lower salt secretion in the hypersalinity site was probably due to lower salt gland frequency induced by xeromorphic characteristics such as smaller, thicker leaves, lower specific area, and thicker cuticles. The ecophysiological data supported the ultrastructural evidence that salt secretion is compromised by prolonged hypersalinity in adult Avicennia marina trees.