Plant-plant interactions play an important role in the structuring of communities and in maintaining genetic diversity in populations. The reciprocal transplant technique is a powerful tool of testing fitness of populations and Connell (1980) suggested field experiments to demonstrate coevolutionary divergence of competitor plant species. This study followed Connell's (1980) proposed experimental design with the aim of determining the occurrence of microevolution among 'Microlaena stipoides' populations with four associated grass species in permanent pastures, namely 'Lolium perenne', 'Poa pratensis' (Ppr), 'Dactylis glomerata' and 'Phalaris aquatica'