Mutants which did not produce extracellular cellulose were found in cultures of two strains of Acetobacter acetigenum (NCIB 8132, NCIB 5346) and one strain of A. xylinum var. africanum (NCIB 7029). With about 150 static cultures of organic NCIB 8132 in a glucose-yeast-extract over a period of about one year, cellulose pellicles were produced in each case. Loss of cellulose-forming ability and the appearance of diffuse general growth were noted in 6 out of 26 tests on static cultures in an ethanol-yeast-extract (EYE) medium, in 3 out of 905 isolates selected from platings of GYE shaken cultures, and in 1 of 3 tests made with EYE shaken cultures. By a mass-plating technique it was found that mutants which did not produce cellulose pellicles were present in the cellulose-producing cultures. The two other cultures (NBIB 5346, NCIB 7029) underwent mutation spontaneously in GYE static cultures.