Mangrove forests are common in subtropical regions, and have received considerable attention as vegetative buffers against anthropogenic N-loading. In this study, we investigated anaerobic ammonium oxidation (anammox) as one of potentially important microbial N-removing pathways in mangrove and shrimp pond sediment in Haiphong, Vietnam. Measurements with (15)N-labeled compounds demonstrated the occurrence of anammox in sediment of mangrove forest and a water channel connecting shrimp ponds to the sea in both 2005 and 2007, and of a semi-intensive shrimp pond in 2005. The rate of potential anammox activity reached to 0.7 nmol-N(2) cm(-3) h(-1), although the contribution of anammox was less significant than denitrification. Anammox-type 16S rRNA gene fragments phylogenetically related to 'Scalindua' species were predominantly recovered from mangrove forest and water channel sediment in a PCR-clone library analysis targeting anammox bacteria. 'Kuenenia'-like gene fragments were also recovered from shrimp pond sediment as the major component. We demonstrated the occurrence of potential anammox activity, and suggested the possibility that diverse species of uncultured anammox bacteria contribute to the nitrogen cycle in subtropical mangrove-aquaculture ecosystems. Furthermore, this study provides new insight into the biogeography of anammox bacteria: 'Scalindua' and 'Kuenenia'-like species coexisted in the blackish sediment as in some temperate estuarine sediment.