Rates of direct denitrification were measured using I5N isotope techniques in intact sediment cores from fringe and basin mangroves in Terminos Lagoon, Mexico. Sediments were injected with 15N0, and the distribution of I5N was measured over time in the head space, overlying water, and sediments. Experiments included an investigation of spatial variation in denitrification rates by comparing results from the fringe and basin mangroves in the rainy season (July 1991); a second experiment was to determine the effect of 3 diffel-ent NO.; concentrations (25, 100, 200 pm01 core-' "N-KN03-) on denitrificatlon rates and was performed in the fringe mangrove during the 'Norte' season (January 1992). Highest I5N-N2 fluxes were measured in the fringe mangrove at 9.4 pm01 m-* h-', while denitrification rates in the basin mangrove ranged from 1.9 to 4.5 pm01 m-' h-'. I5N-Nz fluxes in sediment cores from the fringe mangrove were significantly higher (4.5 to 7.7 pm01 m-2 h-') in cores enriched with 200 pm01 core-' 1 S~-~~0 3 -c o m p a r e d to cores enriched with 25 and 100 pm01 core-' I5N-K N 0 3 -( < l pm01 m-2 h''). Most of the applied I5N was recovered as particulate nitrogen in the sediment and a small fraction reduced to NH,' in both experiments. The low denitrification rates observed In the fringe and basin mangroves indicate that the capacity for sediment denitrification is limited by low NO3-availability. Previous nutrient exchange studies concluded that the fringe mangrove was a 'sink' of NO3-since sediment uptake of NO3-was assumed lost through denitrification. Results from this study show ~1 0 % of sediment N O 3 uptake in fringe mangroves may be lost to denitrification; the remainder being immobilized in the sediment.