Quincy-based Colonial Federal Savings Bank has completed its partial conversion to a stock bank, and the company’s stock began trading on the Nasdaq exchange today.

As part of its first-step conversion from a mutual to a stock bank, Colonial Federal Savings has reorganized into a stock holding company called CFSB Bancorp Inc. and a federally chartered mutual holding company called 15 Beach.

CFSB Bancorp raised gross proceeds of $28 million in its initial public offering, the bank said in a statement. The company sold 2,804,306 shares of common stock at $10 per share. The shares, which represent 42 percent of the company’s stock, trade under the ticker symbol CFSB.

The bank’s mutual holding company, named 15 Beach after the bank’s Quincy address, owns 55 percent of the stock, or about 3.58 million shares. Another 130,433 shares were donated to a newly formed charitable foundation, Colonial Federal Savings Bank Charitable Foundation Inc. The bank also donated $250,000 to the foundation, according to a filing with the SEC.

Colonial Federal Savings had about $336 million in assets as of Sept. 30 and branches in Quincy, Weymouth and Holbrook.

In securities filings, Colonial Federal Savings said that while the bank had capital in excess of all applicable regulatory requirements, the reorganization would support future growth and profitability.

“The reorganization and the capital raised in the offering are expected to provide us with additional capital to support new loans and higher lending limits, support the growth of our banking franchise, provide an additional cushion against unforeseen risks and expand our asset and deposit base,” the bank said in its prospectus.