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The ability of xylophagous organisms such as termites to subsist on a diet containing very little nitrogen has puzzled biologists for a long time. Many studies using the acetylene reduction assay showed that termites are able to fix atmospheric nitrogen with the aid of facultative anaerobic bacteria in their hindguts [6-12]. Experiments with 15N2 gas offered an elegant verification of the assay, confirming the transfer of nitrogen from gut microbes to termite tissues in a caste-selective manner [13]. Here, we apply the 15N natural abundance method, which has been used in estimat-