Summary. The mutagenisity of NTG (N-methyl-N-nitro-N-nitrosoguanidine) for Gloeotrichia ghosei, a cyanobacterium, was enhanced by simultaneous treatment with NTG and chloramphenicol with minimal effect on survival. Addition of chloramphenicol at the time of NTG treatment enhanced the mutation frequency of the fil5marker considerably (about ten times).