Abstract.Growth and gas exchange measurements are used to formulate a carbon balance model to describe the sorghum‐Striga hermonthicahost‐Parasite association.S. hermonthicareduces the growth and radically alters the architecture of infected sorghum plants. Grain and stem weight are reduced, whilst leaf and root biomass are maintained. Losses in host productivity result from two processes: export of carbon to the parasite and Parasite‐induced reductions in host photosynthesis. The latter occurs before the emergence of the Parasite above ground and accounts for 80% of the Predicted loss in production over the lifecycle of the association.S. hermonthicais dependent on carbon exported from the host, since the plant has low rates of photosynthesis coupled with high rates of respiration. Host‐derived carbon accounts for approximately one‐third of the total parasite carbon re