The Illinois State League (ISL) was a baseball minor league formed in 1947. The league operated in 1947-1948 and was a predecessor of today's Midwest League. The Class D ISL was composed entirely of new franchises, each located in Southern Illinois. The charter franchises were in the Illinois cities of Belleville, Centralia, Marion, Mattoon, Mount Vernon and West Frankfort. The ISL was renamed the Mississippi–Ohio Valley League after the 1948 season as the Marion Indians moved to the State of Kentucky to become the Paducah Chiefs. Subsequently, the Mississippi-Ohio Valley League operated through 1955 and in 1956 changed names to become the Midwest League.