Rolla is a city in, and the county seat of, Phelps County, Missouri, United States. Its population in the 2020 United States Census was 19,943. …Rolla is a city in, and the county seat of, Phelps County, Missouri, United States. Its population in the 2020 United States Census was 19,943. It is approximately midway between St. Louis and Springfield along I-44. Its micropolitan statistical area consists of Phelps County, Missouri. Nearby is an inactive township.
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