We analyzed the microbiota of domestic ropy fermented milk, Caspian Sea yogurt (or 'kasupikai yohguruto' in Japanese), circulated in Japan. We collected six varieties from five localities. Lactococcus (L) lactis ssp. cremoris was isolated from all samples as the dominant strain at levels of 10(8)-10(9) CFU/g. We show this strain produces an extracellular polysaccharide (EPS) that causes the unique characteristic viscosity of the product. From analysis of the RAPD pattern of 60 bacterial isolates from the six samples, we found that 59 strains from a total of 60 isolates were identical and produced this viscosity. Furthermore, PFGE analysis of representative strains from each sample indicated that the isolates could be classified into four subgroups. This suggests these L. lactis ssp. cremoris strains found in Caspian Sea yogurt may have been slightly mutated during subculture in Japan. In addition, Lactobacillus (L.) sakei ssp. sakei were isolated from three samples; L. plantarum, Gluconoacetobacter sacchari and Acetobacter aceti were isolated from two samples; and L. paracasei, L. kefiri, Leuconostoc (Leu.) mesenteroides were isolated from one sample.