The unexpected observation of 200 to 400 coliform bacteria per 100 ml in an unpolluted pristine stream was studied within Grand Teton National Park, Wyo. The high numbers of waterborne bacteria occurred in mid- to late summer at a location where there was a coincidental bloom of an algal mat community. Periphyton samplers were used to measure the algal growth that coincided with the increase in number of bacteria. Laboratory studies followed the growth of various coliform bacteria in the supernatant obtained from a Chlorella culture isolated from the mat community. Mixed natural bacterial populations from the stream and pure cultures of water-isolated fecal and nonfecal coliforms increased by two to three orders of magnitude at 13 degrees C when grown in the algal supernatant. Radioactive algal products were obtained by feeding an axenic Chlorella culture C-labeled bicarbonate under laboratory cultivation at 13 degrees C with illumination. Radioactive organic material from the algae became incorporated into the particulate fraction of pure cultures of coliform bacteria as they reproduced and was later released as they died.