From Tua-Nao, a non-salt fermented soybean food of Thailand, bacteria which can be utilized for the improvement of Natto bacterium by cell fusion technique were screened. Screening was carried out for vitamin B₁₂-producing bacteria which are heat-resistant and lysozyme-sensitive. Tua-Nao samples were collected at Lam Phun, Phrao, Doi Saket and Mae Hong Son in northern Thailand. Bacterial counts in Tua-Nao samples were markedly different in number and the ratio of the number of heat-resistant bacteria to the number of total bacteria was also different. Vitamin B₁₂ content varied and some samples did not contain vitamin B₁₂ at all. The lysozyme sensitivities and vitamin B₁₂-producing ability of bacteria separated from Tua-Nao samples after heating were examined. Among 120 isolates, about 20 were lysozyme-sensitive and one (TN02-13) produced vitamin B₁₂. However the isolate (TN02-13) was a mixture of two bacterial isolates (TN02-13a and TN02-13b) and TN02-13b was found to produce vitamin B₁₂ and to be lysozyme-sensitive. Thus a vitamin B₁₂-producing bacterium was obtained from Tua-Nao. This bacterium was regenerated from a protoplast form into a bacillary form.