From Ancestry.com Wiki
This entry was originally written by Dawn M. Knauft and Carol L. Maki in Red Book: American State, County, and Town Sources.
Naturalization records for Wisconsin have historically been kept in the county courthouses. However, they are now generally being transferred to the respective Area Research Center. As of 2002, all but three counties have transferred these records. Milwaukee and Waukesha county papers are deposited with the respective county historical society. Menominee County was created in 1961 and has no older records. Its records are at the county courthouse. Some naturalizations for La Crosse and all those applied for in federal courts are at the National Archives—Great Lakes Region.