MidCap Growth Fund III (Prospectus Summary): | MidCap Growth Fund III

MidCap Growth Fund III  

Supplement dated September 14, 2012

to the Class R-1, Class R-2, Class R-3, Class R-4 and Class R-5 Shares Prospectus

for Principal Funds, Inc.

dated February 29, 2012

(as supplemented on March 16, 2012, April 23, 2012, June 15, 2012, and July 17, 2012)

This supplement updates information currently in the Prospectus. Retain this supplement with the Prospectus.

FUND SUMMARIES

Under the Principal Investment Strategies heading, delete the first paragraph and substitute:  

Under normal circumstances, the Fund invests at least 80% of its net assets, plus any borrowings for investment purposes, in equity securities of companies with medium market capitalizations (those with market capitalizations similar to companies in the Russell Midcap ^® Growth Index (as of December 31, 2011, this range was between approximately $117 million and $20.51 billion)) at the time of purchase. The Fund invests in growth equity securities; growth orientation emphasizes buying equity securities of companies whose potential for growth of capital and earnings is expected to be above average. The Fund may actively trade portfolio securities.