Compatible pollinations from many different taxa display nonrandom mating. Here we describe a system for examining questions of nonrandom mating in Arabidopsis thaliana. Using this system, we demonstrate that Arabidopsis thaliana displays nonrandom mating between distinct accessions. Statistical analysis of these data demonstrates aspects of both pollen competition and male–female complementarity in these matings. Cytological experiments implicate pollen germination and pollen tube growth rates as possible causal factors in these nonrandom mating efficiencies.