Progenies from a five genotype diallel cross in Lolium perenne L. were studied to assess the pattern of genetic control for seed weight, a number of seedling characters, and two adult plant characters.Variation in seed weight was largely determined by the maternal parent, but some control by the pollen parent was apparent. Gene action was additive, but some specific genetic effects were also present, suggesting possible interaction between the haploid pollen genome with a diploid ovular genome in the triploid endosperm, or alternatively between the developing seed and the female parent plant.Of the seedling characters (total germination, rate of germination, rate of leaf and tiller production, length of fifth leaf, and seedling dry matter production) only length of fifth leaf showed any additive variation which might be easily exploited.Adult plant production showed considerable variation which interacts with the season of assessment. However, the relative order of merit remained the same and only the magnitude of the differences in productivity between the families altered. This response was itself under simple genetic control.