Structural parameters of mangrove forests were measured at 7 geographical locations in Florida (USA), Puerto Rico, Mexico and Costa Rica. Plots (0.1 ha) were subdivided into twenty 5 .times. 10 m plots where all individuals greater than 2.5 cm DBH [diameter at breast height] were measured and recorded. Height of the tallest tree in each plot was measured. The complexity index developed by Holdridge (1967) as an integrative measures that combines floral characteristics (number of species) (s), stand density (d), basal area (b) and height (h) was computed as follows: (s) (d) (b) (h) 10-3. The riverine and basin mangrove forests of the southwestern coast of Florida had considerably taller trees (6-9 m) and larger basal areas (20.3-38.5 m2/ha) than did scrub mangrove growing on the southeastern coast of Florida, where a low canopy (1.0 m), a low basal area (6.0 m2/ha) and a correspondingly low complexity index (1.5) were measured. The riverine forests of the Marismas Nacionales located on the Pacific coast of Mexico had high complexity indices (49.7-73.2) due to the large basal areas (57.8-60.8 m2/ha), tall canopies (16-17 m) and a large number of trees greater than 10 cm diameter (103-145/ha). The entire estuarine system of the Marismas Nacionales acts as a receiving basin for large quantities of freshwater runoff and nutrient-rich sediments from surrounding upland watersheds. Except for local sport fishing, the commerical fishing industry of Florida, Puerto Rico and Costa Rican mangrove waters is not nearly as active as that observed near Teacapan, Mexico. The structural characteristics of the north- and south-coast mangroves of Puerto Rico are probably best described by the direct and indirect influences of the climate at each coastline. The basin mangrove forest on the humid north coast (annual rainfall, 1631 mm) has a basal area of 17.8 m2/ha, canopy height of 13.6 m and a complexity index of 16.7. The driest site (rainfall of 860 mm/yr) on the south coast of Puerto Rico was a red mangrove fringe forest, which had a complexity index of 0.9, low canopy height (7 m) and low basal area (6.9 m2/ha). Here upland runoff, draining into the mangroves through porous limestone outcrops, only occurs 1-2 mo. annually. In Costa Rica the riverine mangroves near Puerto Limon on the Caribbean coast (annual rainfall, 3300 mm) are more structurally developed than either the riverine or fringe mangroves of the drier (1800 mm annual rainfall) Pacific coast sites. A basal area of 96.4 m2/ha (primarily Pterocarpus officinalis Jacq.) and a tall canopy (16 m) gave this forest the largest complexity index (84.5) of all forests inventoried. The Pacific coast mangroves of Costa Rica are exposed to seasonal rainfall (6 mo. dry season), and this drier environment was reflected in a shorter canopy (9.5-10.0 m) and a lower basal area (23.2-32.9 m2/ha).