Using a diallel analysis with five parental 'Arabidopsis thaliana ' ecotypes (Col, L'er', Cvi, Ws, and C24), I evaluated inheritance of 12 hybrid traits. Hybrids with C24 often exceeded high-parent values for flowering time, biomass, and reproductive traits. However, some C24 hybrids did not exhibit heterosis - for example C24 with Col for reproductive traits. C24 has a functional FRIGIDA (FRI) allele, while Col, Cvi and Ws have strong Flowering Locus C (FLC) alleles. Functional FRI and FLC act epistatically to delay flowering. Col and L'er' lines introgressed with FRI and FLC alleles and hybrids between these lines and C24 were evaluated. Variation at FRI and FLC affected traits in all backgrounds, but with genotype specific effects. A number of hybrid traits in C24 x Col can be fully accounted for by an epistatic interaction of FRI and FLC. Thus, epistasis between two genes can have a major contribution to heterosis.