He chose Mitchell's Lake, 10 miles south of San Antonio, as the site for his new improved bat roost. "No swamp in the low lands could be worse," Campbell later wrote. All of the city's sewage flowed into the lake and seepage ponds created perfect mosquito breeding conditions. It was a place where travelers were "compelled to whip up their teams to escape the onslaughts of mosquitoes." The tenant farmers who occupied the land surrounding the lake lived with such conditions from spring through fall. Mosquitoes bred in such numbers that at times the farmers were driven from their fields, leaving crops to ruin. Their livestock suffered as well; cows were emaciated and produced little milk, and chickens had pale combs and laid no eggs. Hardly a family escaped malarial infection, and two to four deaths occurred each year. In the spring of 1911, the year Campbell's new bat tower was built, he examined 87 adults and children living around the lake. Seventy-eight had malaria. //

In 1914, four years after the Mitchell's Lake roost was built, duck hunters told Campbell that they could now remain in their blinds until after dark because there were so many less mosquitoes. That summer Campbell began collecting testimonials from the tenant farmers around Mitchell's Lake. They all reported much the same thing: they could now irrigate their fields at night without hoards of mosquitoes attacking them, their work animals were healthy, and there had been no sickness in their families. Campbell didn't find a single case of malaria where four years before 89% of those tested had the disease.

During that same year, the Bexar County Medical Society endorsed Campbell's work, and on June 8, 1914 the City Council of San Antonio passed an ordinance making it unlawful for anyone to kill a bat within the city limits, levying fines from $5 to $200 for each bat killed. Soon after, they appropriated $3,000 of City funds to build another bat tower, the first Municipal Bat Roost in San Antonio or anywhere else. Not to be outdone, the Texas State Board of Health also endorsed Campbell's work and passed their recommendations on to the State Legislature to make it a misdemeanor to kill bats within the entire state. The Governor signed it into law on March 10, 1917.

The original Mitchell's Lake bat tower gained such fame that Campbell opened the area to the public, providing seats and picnic benches for people to watch the evening emergence in comfort. Several more towers were built around San Antonio, and eventually a total of 16 were erected from Texas to Italy, the last one in 1929. The guano harvest from the Mitchell's Lake roost in 1921 was 4,558 pounds, two tons of dry, ready-to-use bat guano with almost double the nitrogen content than cave guano, an amount repeated year after year. At its peak, Campbell estimated that the Mitchell's Lake roost contained over a quarter of a million bats. //

In the mid-1950's, rabies hysteria gripped Texas and bats were taken off the State's protected species list. The end of the bat roost that had once gained the admiration of the world passed without much notice.