Cytokinins are important regulators of growth and development in lower and higher eukaryotic plants. Genetic analysis by means of somatic hybridization, achieved through protoplast fusion, revealed that, of 15 independently isolated gametophore and cytokinin over-producing (OVE) mutants in the model system, Physcomitrella patens, 14 carry recessive mutations responsible for this abnormal phenotype. Seven of these strains have been assigned to three complementation groups: OVEA, OVEB and OVEC. A further three strains have been demonstrated not to belong to the OVEA group and another mutant does not fall into group OVEB. Phenotypic segregation ratios among progeny obtained following self-fertilization of a number of different somatic hybrids showed that several OVE mutations behave as recessive alleles of single Mendelian genes.