Bacterial productivity ((3)H-thymidine incorporation into DNA) and intertidal microbenthic communities were examined within five mangrove estuaries along the tropical northeastern coast of Australia. Bacteria in mangrove surface sediments (0-2 cm depth) were enumerated by epifluorescence microscopy and were more abundant (mean and range: 1.1(0.02-3.6)×10(11) cells·g DW(-1)) and productive (mean: 1.6 gC·m(-2)· d(-1)) compared to bacterial populations in most other benthic environments. Specific growth rates (¯x=1.1) ranged from 0.2-5.5 d(-1), with highest rates of growth in austral spring and summer. Highest bacterial numbers occurred in winter (June-August) in estuaries along the Cape York peninsula north of Hinchinbrook Island and were significantly different among intertidal zones and estuaries. Protozoa (10(5)-10(6)·m(-2), pheopigments (0.0-24.1μg·gDW(-1)) and bacterial productivity (0.2-5.1 gC·m(-2)·d(-1)) exhibited significant seasonality with maximum densities and production in austral spring and summer. Algal biomass (chlorophylla) was low (mean: 1.6μg·gDW(-1)) compared to other intertidal sediments because of low light intensity under the dense forest canopy, especially in the mid-intertidal zone. Partial correlation analysis and a study of possible tidal effects suggest that microbial biomass and bacterial growth in tropical intertidal sediments are regulated primarily by physicochemical factors and by tidal flushing and exposure. High microbial biomass and very high rates of bacterial productivity coupled with low densities of meiofaunal and macroinfaunal consumers observed in earlier studies suggest that microbes may be a sink for carbon in intertidal sediments of tropical mangrove estuaries.