An outbreak of gastroenteritis due to Salmonella Typhimurium occurred among residents of the City of Alamosa, Colorado. The investigation showed that there was no common food source among the ill. The majority of the ill lived within the city limits. There were several infants among the ill who had consumed only formula mixed with tap water. Laboratory results detected Salmonella Typhimurium in the municipal water. Tests of the non-chlorinated artesian well, which fed the water supply, showed no contamination. Tests of water from the distribution lines were contaminated. Residents were ordered to use bottled water until the water system was flushed. A leaky, in-ground, water storage tank was blamed for the contamination; it was known to have been a problem since 1997. Giardia and Cryptosporidium were also found in the city's water, but were not linked to any reported illnesses.