Laboratory tests were conducted on the possible bacteriological effects of combined sewer storm overflows into a tidal estuary. A standard overflow mixture was diluted with 3 to 25 volumes of estuarine waters to represent dilutions expected under average storm conditions. Bacterial growths were observed in the raw overflow and at the various dilutions, using 3-1 flasks at 68○ F, with incubation periods up to 120 hr. MPN values of coliforms were determined after fixed time intervals. Without chlorination MPN values increased from 10 to 40 times the value at the point of discharge, or up to 13,000,000 coliforms per 100 ml after 30 hr of mixture with estuarine water. Chlorination to 15-min chlorine demand of the overflow liquid did not stop aftergrowth but reduced the aftergrowth by 70% to 90%, with peak values of 500,000 per 100 ml in 40 hr. These studies indicate that the aftergrowth phenomenon must be studied in the determinations of design criteria for the treatment of storm overflows from combined sewer systems.