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☑ UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - Q Quarterly report pursuant to Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 For the quarterly period ended September 30 , 2021 OR ☐ Transition report pursuant to Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 For the transition period from to COMMISSION FILE NUMBER 001-34653 FIRST INTERSTATE BANCSYSTEM , INC . ( Exact name of registrant as specified in its charter ) Montana ( State or other jurisdiction of incorporation or organization ) 401 North 31st Street Billings , MT ( Address of principal executive offices ) 81-0331430 ( IRS Employer Identification No. ) 59116-0918 ( Zip Code ) Registrant's telephone number , including area code : ( 406 ) 255-5390 Securities registered pursuant to Section 12 ( b ) of the Act : Title of each class Class A common stock , no par value Trading Symbol ( s ) . FIBK Name of each exchange on which registered NASDAQ Indicate by check mark whether the registrant ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . Yes ☑ No □ Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( §232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files . ) Yes ☑ No □ Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company , or an emerging growth company . See the definitions of “ large accelerated filer , " " accelerated filer , ” “ smaller reporting company ” and “ emerging growth company " in Rule 12b - 2 of the Exchange Act . Large accelerated filer Non - accelerated filer ☑ ☐ ☐ ☐ Accelerated filer Smaller reporting company Emerging growth company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . Indicate by check mark whether the registrant is a shell company ( as defined in Rule 12b - 2 of the Exchange Act ) . Yes □ No ☑ Indicate the number of shares outstanding of each of the registrant's classes of common stock , as of the latest practicable date : September 30 , 2021 - Class A common stock September 30 , 2021 - Class B common stock 41,715,627 20,515,516