Somatic cells of Armillaria gallica fruit bodies have been shown to possess different genotypes for molecular-marker and mating-type loci. Here we report experiments on six quantitative traits and demonstrate that somatic cells of fruit bodies possess almost as much genetic variation for growth rate and phenotypic plasticity as do spores, the products of meiosis. Genetically distinct somatic cells therefore have the potential to grow at different rates relative to one another during primordial fruit body formation. This may confer an advantage on all cell lines within a fruit body, not just those that happen to grow better under a particular set of conditions. To our knowledge, genetic variation for fitness-related traits that make up a single genetic individual has not been reported before.