The diet of the harvester termite Hodotermes mossambicus was investigated at two sites with distinct dietary components: C₄ grasses (δ¹³C isotope values, −13.8‰ to −14.0‰) and C₃ plants (δ¹³C isotope values, −25.6‰ to −27.1‰). By comparing observations of food items carried into the colony by the termites and carbon isotope ratios of whole termites (that determined assimilated carbon), the relative proportion of the C₃ and C₄ plant food components of the termite diet was estimated. There was agreement between the observational data and stable carbon isotopic data, with grass representing approximately 93% of the diet of H. mossambicus at two study sites (urban and rural) on the South African highveld. However, when correcting for mass of food items, that is, C₃ and C₄, carried by termites, the proportion of grass (C₄) in the diet may be underestimated.