To investigate the cost of the dimorphic cyanogenic system in Trifolium repens L., genotypes of the cyanogenic (Tc) and acyanogenic (Ta) morphs were grown in mixtures over a range of ratios and in pure stands at two levels of total density (low in a first experiment, high in a second experiment). The principles of the competition analysis employed were those related to the inverse linear model response. The morphs were compared using two interaction indices, the substitution rate and the relative resource total (RRT). The relative fitness of the two morphs, i.e. biomass and number of flowers per plant, suggests that the Ta morph has a competitive advantage over the Tc morph.