First Interstate BancSystem (FIBK) 10-K 2007 FY Annual report

Exhibit 12.1

Computation of Ratio of Earnings to Fixed Charges
(Dollars in thousands)

Year ended December 31,	2007			2006			2005			2004			2003
Earnings:
Net income from operations after tax	$	68,641		$	75,609		$	54,715		$	45,421		$	40,752
Applicable income taxes		36,793			41,499			29,310			23,929			22,267
Income before taxes		105,434			117,108			84,025			69,350			63,019
Fixed charges:
Interest expense excluding interest on deposits		5,193			7,415			5,360			4,397			5,000
Portion of rents representative of interest		—			—			194			293			325
Amortization of subordinated debenture/trust preferred securities issuance costs		323			37			41			44			961
Fixed charges excluding interest on deposits		5,516			7,452			5,595			4,734			6,286
Interest on deposits		120,761			98,545			58,189			38,024			43,614
Fixed charges including interest on deposits	$	126,277		$	105,997		$	63,784		$	42,758		$	49,900
Earnings before fixed charges excluding interest on deposits	$	110,940		$	124,560		$	89,620		$	74,084		$	69,305
Earnings before fixed charges including interest on deposits		231,711			223,105			147,809			112,108			112,919
Fixed charges excluding interest on deposits		5,516			7,452			5,595			4,734			6,286
Fixed charges including interest on deposits		126,277			105,997			63,784			42,758			49,900
Ratio of Earnings to Fixed Charges:
Excluding interest on deposits		20.11	x		16.71	x		16.02	x		15.65	x		11.03	x
Including interest on deposits		1.83	x		2.10	x		2.32	x		2.62	x		2.26	x