1. In the current ecological classification of termites, four feeding groups (I–IV) are recognised, corresponding to a gradient of decomposition from sound wood to highly mineralised organic matter in the soil.
2. Nitrogen stable isotopes (hereafter δ15N) were used to place termites from French Guiana rainforests along a wood-soil decomposition gradient, to test (i) whether feeding group assignation based on morphological characters was accurate and actually represented diet specialisation thresholds, and (ii) to what extent the dietary specialization of species is explained by phylogeny (phylogenetic autocorrelation).
3. δ15N values vary over a range of 13‰, suggesting that diet diversification contributes to the high species diversity in French Guiana. δ15N values span a similar interval in all Termitidae subfamilies. Ranges of different subfamilies broadly overlap, although each of them diversified preferentially on one side of the wood-soil decomposition gradient. Congeneric species share similar feeding habits, whereas distant species tend to feed on distinct substrates.
4. Feeding groups did not completely match stable isotope data: there was no discontinuity between Groups III and IV, and no correlation between anatomical criteria used to distinguish these groups and δ15N values. Nor was there any consistent difference in δ15N values between wood feeders of the families Rhinotermitidae (Group I) and Termitidae (Group II). We also suggest that species feeding outside the wood-soil gradient should be distinguished for their peculiar feeding requirements.