According to reports hitherto on the isolation of Vibrio cholerae non-O1 from the environment, environmental conditions at the time of detection differ depending on whether isolation is from the sea or rivers. We based this study on two rivers of differing water quality and studied the trend of a bacteria indicator to observe the conditions of existence of V. cholerae non-O1. 1) It was thought that V. cholerae non-O1 did not flow from the land or other areas of pollution but existed at those points in the river from which they were isolated. 2) The proportion of V. cholerae non-O1 isolated was higher for the river with a higher degree of water pollution and higher in areas of high pollution in both rivers. 3) In rivers with a low degree of pollution or having a small range in water quality, an increase in Cl- and PO4(3-) ions, or an increase in the number of bacteria grown on nutrient agar medium at 37 degrees C for 24 hours appears to increase the frequency of isolation of V. cholerae non-O1. 4) Approximately 13% of the samples of isolated V. cholerae non-O1 exhibited the cholera enterotoxin strain.