Abscisic acid (ABA) controls plants’ tolerance to environmental stress such as freezing and desiccation. We previously reported that AR7, the ABA-insensitive mutant of Physcomitrella patens has a defect in the ARK gene encoding the group B3 MAP kinase kinase kinase, with a single amino acid change from serine to phenylalanine in the non-kinase domain having a putative regulatory function. By ultraviolet mutagenesis, we obtained over one hundred ABA-insensitive mutants, from which two ark nonsense mutants were identified. These mutants showed inhibited protonemal growth with elongated gametophore in the absence of ABA, in contrast to AR7 showing protonemal growth similar to wild type. These results indicate that ARK is involved in regulation of plant growth in addition to stress responses.