Aims: To evaluate the water quality of recently flooded tube wells in Bangladesh and the effect of spot chlorination on improving bacteriological quality. Methods and Results: The study team identified and tested water samples from 127 tube wells that were flooded within the preceding 4 weeks. Twenty-six of the tube wells with the highest concentration of thermotorlerant coliform bacteria were randomly assigned to spot chlorination vs control. On initial screening, water samples from 56 recently inundated tube wells (44%) were contaminated with thermotolerant coliforms. Among the 13 wells randomized to chlorination, there was no change in the proportion of water samples that had no detectable thermotolerant coliform bacteria immediately before chlorine treatment (n = 4, 23%) and 60 min following chlorine treatment (n = 4, 23%). Similarly, there was no difference in the proportion of water samples that had no detectable thermotolerant coliforms between chlorine spot treated and control tube wells 7-18 days later (31 vs 23% P = 0-66). Conclusions: Spot chlorine treatment of inundated tube wells in Bangladesh three to 6 weeks after the flooding did not improve drinking water quality. Significance and Impact of the Study: Unless modified methods improve effectiveness, resources should not be spent promoting spot chlorination of flooded tube wells.