Wild-type strains of the cyanobacterium Planktothrix rubescens have a cluster of gas vesicle (gvp) genes with repeats of alternating gvpA and gvpC. The gvpC occurs in three length variants, all with the same 3'-sequence, OmegaC. Spontaneous non-buoyant mutants had lost some of the alternating gvpAC copies and their gvpC genes had a novel 3'-end sequence, PsiC; additional gvpC genes terminating in this sequence were also found in the wild-type and representatives of other GV genotypes. Alleles of gvpC terminating in PsiC occurred only at the downstream ends of the gvpAC clusters investigated; all other gvpCs terminated in OmegaC. Mutants of strains with the GV3 genotype produced only 30-50% of the gas vesicles present in the wild-type; their gas vesicles had lower mean critical pressures (0.70-0.78 MPa) than those in the wild-type (1.05-1.10 MPa).