Plant invasions can fundamentally alter detrital inputs and the structure of detritus-based food webs. We examined the detrital pathways in mangrove food webs in native (Puerto Rican) and introduced (Hawaiian) Rhizophora mangle forests using a dual isotope approach and a mixing model. Based on trophic-level fractionation of 0-1[per thousand] for d ^sup 13^C and 2-3[per thousand] for d ^sup 15^N, among the invertebrates, only nematodes, oligochaetes, and nereid polychaetes from native mangroves exhibited stable isotopes consistent with a mangrove-derived diet. Certain fauna, in particular tubificid oligochaetes, had d ^sup 13^C values consistent with the consumption of mangrove leaves, but they were depleted in ^sup 15^N, suggesting their primary nitrogen source was low in ^sup 15^N, and was possibly N^sub 2^-fixing bacteria. In introduced mangroves, all feeding groups appeared to rely heavily on non-mangrove sources, especially phytoplankton inputs. Mixing model results and discriminant analysis showed clear separation of introduced and native mangrove sites based on differential food source utilization within feeding groups, with stronger and more diverse use of benthic foods observed in native forests. Observed differences between native and invasive mangrove food webs may be due to Hawaiian detritivores being poorly adapted to utilizing the tannin-rich, nitrogen-poor mangrove detritus. In addition, differential utilization of mangrove detritus between native and introduced mangroves may be a consequence of forest age. We postulate that increasing mangrove forest age may promote diversification of bacterial food webs important in N and S cycling. Our results also suggest a potentially important role for sulfur bacteria in supporting the most abundant infaunal consumers, nematodes, in the most mature systems.