AbstractAfter screening large numbers of survivors of mutagenic treatment with MNNG, 165 strains of the mossPhyscomitrella patensaltered in stem or leaf development were isolated. Of these, seven strains which produce leaves of greatly reduced size and cell number were analysed genetically, using protoplast fusion to obtain hybrids for dominance and complementation tests, since the strains are sterile. Three of the mutant alleles are recessive to the wild-type allele, but this is not true of the other four. Four of the mutant alleles can be assigned to one complementation group, and two to a second group.